feat(wallet): denominate invoices in any currency the server accepts

Receiving was sats-only. An invoice can now be quoted in any currency
the LNbits instance allows (~165 by default, narrowable via
`LNBITS_ALLOWED_CURRENCIES`), which is what a merchant pricing in EUR
actually needs.

LNbits does the conversion: `unit: "EUR"` with `amount: 5.50` yields an
ordinary BOLT11 invoice for the equivalent sats and records the original
denomination on the payment's `extra` (`fiat_currency`, `fiat_amount`,
`fiat_rate`). Nothing about the payment rail changes — the payer still
settles over Lightning.

- `useCurrencies` (base module) fetches the currency list and the
  instance default once per process, de-duplicating concurrent callers.
  It sits beside the other shared payment primitives because four
  modules already duplicate this same `getCurrencies()` call; they can
  adopt it later.
- Decimal places come from `Intl.NumberFormat`, not a hardcoded table,
  so the input step is 0.01 for EUR and 1 for JPY, which has no minor
  unit. Amount validation tracks the unit too.
- Switching unit clears a typed amount. Reinterpreting "10" from sats to
  EUR would silently create an invoice for a wildly different value.
- The created invoice leads with what the payer was quoted (€5.50) and
  shows the sat amount as secondary, noting the rate was fixed at
  creation.
- A live "≈ N sats" preview uses the shared conversion composable. It is
  best-effort: a failed lookup renders nothing and never blocks
  creation, since LNbits converts authoritatively server-side.
- Sats stays the default, and the selector is hidden entirely if the
  currency list is unavailable, so the wallet still works if the rate
  service is down.

Verified in a headless browser against the live LNbits:

  selector lists 166 options (sats + 165 currencies)
  EUR 5.50 -> preview "≈ 7,301 sats" -> POST {amount: 5.5, unit: "EUR"}
           -> invoice lnbc73010n... (7301 sats), shown as "€5.50"
              with "Payable as 7,301 sats"
  sats 250 -> POST {amount: 250, unit: "sat"}, no fiat line
  JPY step/min 1; EUR step/min 0.01
  switching EUR -> JPY clears the typed "12.34"

Fiat in the transaction history is deliberately left out; it applies to
all payments rather than just newly created ones, and is tracked
separately.

src/lib/api/lnbits.ts

@@ -226,6 +226,27 @@ export class LnbitsAPI extends BaseService {
     })
   }
 
+  /**
+   * Currency codes this LNbits instance accepts for fiat-denominated
+   * amounts (ISO 4217). Unrestricted instances return all ~165; an admin
+   * can narrow it via `LNBITS_ALLOWED_CURRENCIES`.
+   */
+  async getCurrencies(): Promise<string[]> {
+    return this.request<string[]>('/currencies', { method: 'GET' })
+  }
+
+  /**
+   * The instance's default accounting currency, or `null` when the admin
+   * has not configured one.
+   */
+  async getDefaultCurrency(): Promise<string | null> {
+    const data = await this.request<{ default_currency: string | null }>(
+      '/default-currency',
+      { method: 'GET' }
+    )
+    return data.default_currency ?? null
+  }
+
   isAuthenticated(): boolean {
     return !!this.accessToken
   }

src/modules/base/composables/useCurrencies.ts

@@ -0,0 +1,110 @@
+import { ref, computed, type Ref } from 'vue'
+import { injectService, SERVICE_TOKENS } from '@/core/di-container'
+import type { LnbitsAPI } from '@/lib/api/lnbits'
+
+/**
+ * Currency codes accepted by this LNbits instance, plus the instance default.
+ *
+ * Both are server configuration that cannot change within a session, so the
+ * fetch is shared process-wide and de-duplicated: a dozen components calling
+ * this produce one request, not a dozen. Concurrent callers await the same
+ * in-flight promise rather than racing.
+ *
+ * Failure is non-fatal. `currencies` stays empty and callers fall back to
+ * sats-only, which keeps the wallet usable when the rate service is down.
+ */
+
+const currencies = ref<string[]>([])
+const defaultCurrency = ref<string | null>(null)
+const loading = ref(false)
+const error = ref<string | null>(null)
+
+// Shared across every caller so the request happens exactly once.
+let inFlight: Promise<void> | null = null
+let loaded = false
+
+/**
+ * How many decimal places a currency is conventionally written with.
+ *
+ * Fiat is not uniformly two decimals: JPY and KRW have none, and entering
+ * "5.50 JPY" is meaningless. `Intl.NumberFormat` already knows each
+ * currency's convention, so ask it rather than hardcoding a table.
+ */
+export function currencyDecimals(code: string): number {
+  try {
+    const fmt = new Intl.NumberFormat(undefined, {
+      style: 'currency',
+      currency: code,
+    })
+    return fmt.resolvedOptions().maximumFractionDigits ?? 2
+  } catch {
+    // Unknown or non-ISO code — assume the common case.
+    return 2
+  }
+}
+
+/** Format an amount with its currency symbol, in the viewer's locale. */
+export function formatCurrency(amount: number, code: string): string {
+  try {
+    return new Intl.NumberFormat(undefined, {
+      style: 'currency',
+      currency: code,
+    }).format(amount)
+  } catch {
+    return `${amount.toFixed(currencyDecimals(code))} ${code}`
+  }
+}
+
+export function useCurrencies(): {
+  currencies: Ref<string[]>
+  defaultCurrency: Ref<string | null>
+  loading: Ref<boolean>
+  error: Ref<string | null>
+  hasCurrencies: Ref<boolean>
+  load: () => Promise<void>
+  currencyDecimals: typeof currencyDecimals
+  formatCurrency: typeof formatCurrency
+} {
+  const lnbitsAPI = injectService<LnbitsAPI>(SERVICE_TOKENS.LNBITS_API)
+
+  async function load(): Promise<void> {
+    if (loaded) return
+    if (inFlight) return inFlight
+
+    loading.value = true
+    error.value = null
+
+    inFlight = (async () => {
+      try {
+        // The default currency is optional context, so a failure there must
+        // not cost us the currency list itself.
+        const [list, fallback] = await Promise.all([
+          lnbitsAPI.getCurrencies(),
+          lnbitsAPI.getDefaultCurrency().catch(() => null),
+        ])
+        currencies.value = Array.isArray(list) ? list : []
+        defaultCurrency.value = fallback
+        loaded = true
+      } catch (err) {
+        error.value = err instanceof Error ? err.message : 'Failed to load currencies'
+        console.warn('[useCurrencies] load failed:', err)
+      } finally {
+        loading.value = false
+        inFlight = null
+      }
+    })()
+
+    return inFlight
+  }
+
+  return {
+    currencies,
+    defaultCurrency,
+    loading,
+    error,
+    hasCurrencies: computed(() => currencies.value.length > 0),
+    load,
+    currencyDecimals,
+    formatCurrency,
+  }
+}

src/modules/wallet/components/ReceiveDialog.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed } from 'vue'
+import { ref, computed, watch, onMounted } from 'vue'
 import { useForm } from 'vee-validate'
 import { toTypedSchema } from '@vee-validate/zod'
 import * as z from 'zod'
@@ -11,8 +11,19 @@ import { Textarea } from '@/components/ui/textarea'
 import { Label } from '@/components/ui/label'
 import { QrCode, Copy, Check, Loader2 } from 'lucide-vue-next'
 import { FormControl, FormDescription, FormField, FormItem, FormLabel, FormMessage } from '@/components/ui/form'
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from '@/components/ui/select'
+import { useCurrencies } from '@/modules/base/composables/useCurrencies'
+import { usePriceConversion } from '@/modules/base/composables/usePriceConversion'
 import type { Invoice } from '../services/WalletService'
 
+const SAT = 'sat'
+
 interface Props {
   open: boolean
 }
@@ -29,11 +40,47 @@ const walletService = injectService(SERVICE_TOKENS.WALLET_SERVICE) as any
 const toastService = injectService(SERVICE_TOKENS.TOAST_SERVICE) as any
 const paymentService = injectService(SERVICE_TOKENS.PAYMENT_SERVICE) as any
 
-// Form validation schema
+// Denomination currency: "sat", or an ISO 4217 code the server accepts.
-const formSchema = toTypedSchema(z.object({
+const unit = ref<string>(SAT)
-  amount: z.number().min(1, "Amount must be at least 1 sat").max(10000000, "Amount too large"),
+const isFiat = computed(() => unit.value !== SAT)
-  memo: z.string().max(640, "Description too long").optional()
+
-}))
+const {
+  currencies,
+  defaultCurrency,
+  hasCurrencies,
+  load: loadCurrencies,
+  currencyDecimals,
+  formatCurrency,
+} = useCurrencies()
+const { useLivePreview } = usePriceConversion()
+
+onMounted(async () => {
+  await loadCurrencies()
+  // Honour the instance's default accounting currency when the admin set
+  // one and it is actually offered. Otherwise stay on sats.
+  const preferred = defaultCurrency.value
+  if (preferred && currencies.value.includes(preferred)) {
+    unit.value = preferred
+  }
+})
+
+// Sats are whole numbers; fiat follows its own convention (JPY has no
+// decimals, EUR has two), so the step and the minimum both track the unit.
+const amountStep = computed(() =>
+  isFiat.value ? Math.pow(10, -currencyDecimals(unit.value)) : 1
+)
+const amountMin = computed(() => amountStep.value)
+
+// Form validation schema. The floor differs by unit: 1 sat vs 0.01 EUR.
+const formSchema = computed(() =>
+  toTypedSchema(z.object({
+    amount: z
+      .number()
+      .min(amountMin.value, `Amount must be at least ${amountMin.value} ${unit.value}`)
+      .max(10000000, 'Amount too large'),
+    memo: z.string().max(640, 'Description too long').optional()
+  }))
+)
 
 // Form setup
 const form = useForm({
@@ -44,9 +91,28 @@ const form = useForm({
   }
 })
 
-const { resetForm, meta } = form
+const { resetForm, meta, values } = form
 const isFormValid = computed(() => meta.value.valid)
 
+// Live "≈ N sats" preview under a fiat amount, so the payer's quote is not a
+// surprise. Best-effort: a failed conversion renders nothing and never blocks
+// invoice creation, since LNbits does the authoritative conversion server-side.
+const previewAmount = computed(() => (isFiat.value ? Number(values.amount) || 0 : 0))
+const previewFrom = computed(() => unit.value)
+const previewTo = computed(() => SAT)
+const { result: satPreview, loading: satPreviewLoading } = useLivePreview(
+  previewAmount,
+  previewFrom,
+  previewTo
+)
+
+// Switching unit invalidates a typed amount: "10" means something very
+// different in sats than in EUR, and silently reinterpreting it risks an
+// invoice for the wrong value.
+watch(unit, () => {
+  form.setFieldValue('amount', undefined as unknown as number)
+})
+
 // State
 const createdInvoice = ref<Invoice | null>(null)
 const copiedField = ref<string | null>(null)
@@ -96,7 +162,8 @@ const onSubmit = form.handleSubmit(async (formValues) => {
   try {
     const invoice = await walletService.createInvoice({
       amount: formValues.amount,
-      memo: formValues.memo || 'Payment request'
+      memo: formValues.memo || 'Payment request',
+      unit: unit.value
       // Let server use default expiry
     })
 
@@ -197,18 +264,49 @@ function formatExpiry(seconds: number): string {
         <form @submit="onSubmit" class="space-y-3 sm:space-y-4">
           <FormField v-slot="{ componentField }" name="amount">
             <FormItem>
-              <FormLabel class="text-sm font-medium">Amount (sats) *</FormLabel>
+              <FormLabel class="text-sm font-medium">Amount *</FormLabel>
+              <div class="flex gap-2">
                 <FormControl>
                   <Input
                     type="number"
-                  min="1"
+                    :min="amountMin"
+                    :step="amountStep"
                     max="10000000"
-                  placeholder="Enter amount in sats"
+                    :placeholder="isFiat ? `Enter amount in ${unit}` : 'Enter amount in sats'"
-                  class="h-9 sm:h-10"
+                    class="h-9 sm:h-10 flex-1"
                     v-bind="componentField"
                   />
                 </FormControl>
-              <FormDescription class="text-xs sm:text-sm">Amount to request in satoshis</FormDescription>
+                <!-- Sats-only when the server offers no currencies (or the
+                     list failed to load), so the wallet still works. -->
+                <Select v-if="hasCurrencies" v-model="unit">
+                  <SelectTrigger class="h-9 sm:h-10 w-[7.5rem] shrink-0">
+                    <SelectValue />
+                  </SelectTrigger>
+                  <SelectContent>
+                    <SelectItem value="sat">sats</SelectItem>
+                    <SelectItem
+                      v-for="currency in currencies"
+                      :key="currency"
+                      :value="currency"
+                    >
+                      {{ currency }}
+                    </SelectItem>
+                  </SelectContent>
+                </Select>
+              </div>
+              <FormDescription class="text-xs sm:text-sm">
+                <span v-if="!isFiat">Amount to request in satoshis</span>
+                <span v-else-if="satPreviewLoading" class="text-muted-foreground">
+                  Converting…
+                </span>
+                <span v-else-if="satPreview">
+                  ≈ {{ Math.round(satPreview).toLocaleString() }} sats at the current rate
+                </span>
+                <span v-else>
+                  Amount to request in {{ unit }}, converted to sats when the invoice is created
+                </span>
+              </FormDescription>
               <FormMessage />
             </FormItem>
           </FormField>
@@ -267,7 +365,12 @@ function formatExpiry(seconds: number): string {
           <div class="grid grid-cols-1 sm:grid-cols-2 gap-2 sm:gap-4 text-sm">
             <div class="flex justify-between sm:block">
               <span class="text-muted-foreground">Amount:</span>
-              <span class="font-medium sm:ml-2">{{ createdInvoice.amount }} sats</span>
+              <span class="font-medium sm:ml-2">
+                <template v-if="createdInvoice.fiatCurrency && createdInvoice.fiatAmount">
+                  {{ formatCurrency(createdInvoice.fiatAmount, createdInvoice.fiatCurrency) }}
+                </template>
+                <template v-else>{{ createdInvoice.amount }} sats</template>
+              </span>
             </div>
             <div class="flex justify-between sm:block">
               <span class="text-muted-foreground">Status:</span>
@@ -275,6 +378,11 @@ function formatExpiry(seconds: number): string {
             </div>
           </div>
 
+          <div v-if="createdInvoice.fiatCurrency" class="text-xs sm:text-sm text-muted-foreground">
+            Payable as <span class="font-medium text-foreground">{{ createdInvoice.amount.toLocaleString() }} sats</span>,
+            fixed at the rate when this invoice was created.
+          </div>
+
           <div>
             <span class="text-muted-foreground text-xs sm:text-sm">Description:</span>
             <p class="font-medium text-sm break-words">{{ createdInvoice.memo }}</p>
@@ -309,7 +417,13 @@ function formatExpiry(seconds: number): string {
             <Check class="h-12 w-12 sm:h-16 sm:w-16 text-green-600 dark:text-green-400 mb-2 sm:mb-4" />
             <div class="text-center px-2">
               <p class="text-base sm:text-lg font-semibold text-green-800 dark:text-green-200">Payment Received!</p>
-              <p class="text-sm text-green-600 dark:text-green-400 mt-1">{{ createdInvoice.amount }} sats</p>
+              <p class="text-sm text-green-600 dark:text-green-400 mt-1">
+                <template v-if="createdInvoice.fiatCurrency && createdInvoice.fiatAmount">
+                  {{ formatCurrency(createdInvoice.fiatAmount, createdInvoice.fiatCurrency) }}
+                  ({{ createdInvoice.amount.toLocaleString() }} sats)
+                </template>
+                <template v-else>{{ createdInvoice.amount }} sats</template>
+              </p>
             </div>
           </div>
         </div>

src/modules/wallet/services/WalletService.ts

@@ -22,9 +22,16 @@ export interface SendPaymentRequest {
 }
 
 export interface CreateInvoiceRequest {
+  /** Amount expressed in `unit`. Sats when `unit` is "sat", else fiat major units (e.g. 5.50 EUR). */
   amount: number
   memo: string
   expiry?: number // Optional expiry in seconds
+  /**
+   * Denomination currency. "sat" (default) or an ISO 4217 code the server
+   * accepts. LNbits converts a fiat amount to sats at creation time using
+   * its own rate; the result is an ordinary BOLT11 invoice.
+   */
+  unit?: string
 }
 
 export interface Invoice {
@@ -32,10 +39,16 @@ export interface Invoice {
   bolt11: string // The BOLT11 invoice
   payment_request: string // Same as bolt11, for compatibility
   checking_id: string
+  /** Always sats — the settled amount, converted by LNbits when fiat-denominated. */
   amount: number
   memo: string
   time: number
   expiry: number | null
+  /** Set when the invoice was denominated in fiat: the original amount and its currency. */
+  fiatAmount?: number
+  fiatCurrency?: string
+  /** Sats per unit of `fiatCurrency` at creation time, as recorded by LNbits. */
+  fiatRate?: number
 }
 
 export interface PaymentTransaction {
@@ -190,7 +203,9 @@ export default class WalletService extends BaseService {
         body: JSON.stringify({
           out: false, // Incoming payment (receiving)
           amount: request.amount,
-          unit: 'sat',
+          // A fiat unit makes LNbits convert to sats at its own rate and
+          // record the original denomination in the payment's `extra`.
+          unit: request.unit || 'sat',
           memo: request.memo,
           expiry: request.expiry || 3600 // Default 1 hour expiry
         })
@@ -209,7 +224,13 @@ export default class WalletService extends BaseService {
         ...rawInvoice,
         payment_request: rawInvoice.bolt11, // Copy bolt11 to payment_request for compatibility
         amount: rawInvoice.amount / 1000, // Convert from millisats to sats
-        expiry: rawInvoice.expiry ? this.parseExpiryToSeconds(rawInvoice.expiry) : null
+        expiry: rawInvoice.expiry ? this.parseExpiryToSeconds(rawInvoice.expiry) : null,
+        // For a fiat-denominated invoice LNbits echoes the original amount,
+        // currency and rate back on `extra`. Surface them so the UI can show
+        // what the payer was actually quoted rather than only the sat amount.
+        fiatAmount: rawInvoice.extra?.fiat_amount,
+        fiatCurrency: rawInvoice.extra?.fiat_currency,
+        fiatRate: rawInvoice.extra?.fiat_rate
       }
 
       console.log('Processed invoice:', invoice)