fix(core): read the routing fee from fee, not the absent fee_msat

Closes #170.

LNbits payment payloads carry `fee`, signed millisats like `amount`.
There is no `fee_msat` field, so three modules read `undefined`:

  PaymentService.PaymentResult declared `fee_msat: number` — a field the
  API never sends — so the compiler vouched for a number that was always
  undefined, and the lie propagated to every consumer.

  market/useLightningPayment persisted `feeMsat: undefined` onto paid
  orders (on a property the Order type does not even declare).

  events/LnbitsPaymentProvider had `data.fee_msat ?? 0`, whose fallback
  masked the miss so every payment recorded a zero fee.

`payInvoice` now normalizes at the boundary instead of returning the raw
payload: it maps `fee` to a positive `feeMsat`. Doing it once there means
no call site can pick the wrong field name or forget that outgoing
amounts are signed. `PaymentResult.feeMsat` replaces the phantom field,
so the type finally matches what callers receive.

Verified against a live pay-invoice response, plus a synthetic payment
carrying a real routing fee (internal LNbits transfers are fee-free, so
the live case cannot exercise a non-zero fee):

  live response       fee present: True   fee_msat present: False
  synthetic -1234 msat
    old (events)  -> 0      fee silently lost
    old (market)  -> undefined
    new (both)    -> 1234   positive magnitude

Nothing read these values downstream, so no totals were miscomputed; the
fee was simply recorded as missing or zero.

src/core/services/PaymentService.ts

@@ -5,7 +5,15 @@ import { toast } from 'vue-sonner'
 
 export interface PaymentResult {
   payment_hash: string
-  fee_msat: number
+  /**
+   * Routing fee in millisats, always positive.
+   *
+   * LNbits returns this as `fee`, not `fee_msat`, and signs it negative for
+   * outgoing payments (like `amount`). This interface previously declared
+   * `fee_msat: number`, a field the API never sends, so every consumer read
+   * `undefined` while the compiler vouched for a number.
+   */
+  feeMsat: number
   preimage: string
 }
 
@@ -230,7 +238,16 @@ export class PaymentService extends BaseService {
       throw new Error(errorMessage)
     }
 
-    return await response.json()
+    // Normalize at the boundary rather than handing the raw LNbits payload
+    // to callers: `fee` is signed millisats, and consumers all want a
+    // positive magnitude. Doing it here means no call site can pick the
+    // wrong field name or forget the sign.
+    const raw = await response.json()
+    return {
+      payment_hash: raw.payment_hash,
+      feeMsat: Math.abs(Number(raw.fee ?? 0)),
+      preimage: raw.preimage ?? ''
+    }
   }
 
   /**
@@ -266,7 +283,7 @@ export class PaymentService extends BaseService {
       
       this.debug('Payment successful', {
         paymentHash: paymentResult.payment_hash,
-        feeMsat: paymentResult.fee_msat
+        feeMsat: paymentResult.feeMsat
       })
 
       // Show success notification if enabled

src/modules/events/services/LnbitsPaymentProvider.ts

@@ -84,7 +84,9 @@ export class LnbitsPaymentProvider implements PaymentProvider {
     const data = await response.json()
     return {
       paymentHash: data.payment_hash,
-      feeMsat: data.fee_msat ?? 0,
+      // LNbits sends `fee` (signed millisats), never `fee_msat`. Reading the
+      // latter meant every payment recorded a zero fee.
+      feeMsat: Math.abs(Number(data.fee ?? 0)),
       preimage: data.preimage ?? '',
     }
   }

src/modules/market/composables/useLightningPayment.ts

@@ -42,7 +42,7 @@ export function useLightningPayment() {
             status: 'paid' as const,
             paidAt: Math.floor(Date.now() / 1000),
             paymentHash: paymentResult.payment_hash,
-            feeMsat: paymentResult.fee_msat,
+            feeMsat: paymentResult.feeMsat,
             items: [...order.items], // Convert readonly to mutable
             shippingZone: order.shippingZone ? {
               ...order.shippingZone,
@@ -88,7 +88,7 @@ export function useLightningPayment() {
             status: 'paid' as const,
             paidAt: Math.floor(Date.now() / 1000),
             paymentHash: result.payment_hash,
-            feeMsat: result.fee_msat,
+            feeMsat: result.feeMsat,
             items: [...order.items], // Convert readonly to mutable
             shippingZone: order.shippingZone ? {
               ...order.shippingZone,