fix(base): resolve the sats key in price conversions

Every fiat-to-sat conversion returned null. `convert()` looked up the
response by the `to` key it had passed in, but LNbits does not echo that
key: it always names the satoshi amount "sats" (plural), whatever the
caller asked for. So `data["sat"]` missed and the function fell through
to its null path.

    POST {from_: "EUR", to: "sat", amount: 5.50}
      -> {"EUR": 5.5, "sats": 7298, "BTC": 7.298e-05}

This silently broke the one shipped caller that converts in that
direction: `PurchaseTicketDialog` computes `lightningSats` via
`convert(amount, currency, 'sat')`, so it was always null.
`PriceConversionPreview` has a `to === 'sat'` formatting branch that
could never be reached either.

Key selection moves into `pickConverted`, which maps sat/sats onto the
plural key the server actually uses and keeps the previous fallbacks.
The reverse direction (sat to fiat) already worked and is unchanged.

Verified against the live LNbits:

  EUR -> sat (5.50):  old=null   new=7299     FIXED
  USD -> sat (10):    old=null   new=11588    FIXED
  JPY -> sat (1000):  old=null   new=7360     FIXED
  sat -> EUR (7295):  old=5.4969 new=5.4969   unchanged

src/modules/base/composables/usePriceConversion.ts

@@ -21,6 +21,37 @@ function cacheKey(amount: number, from: string, to: string): string {
  * conversion preview as best-effort UX, never as a blocker. 60s in-memory
  * cache de-duplicates dialog re-renders.
  */
+/**
+ * Pull the converted value out of an LNbits `/conversion` response.
+ *
+ * The endpoint does not echo the `to` key it was given. It always names the
+ * satoshi amount `"sats"` (plural) regardless of whether the caller asked for
+ * "sat" or "sats", and it returns fiat under the upper-case ISO code:
+ *
+ *   POST {from_: "EUR", to: "sat",  amount: 5.50}  -> {EUR: 5.5, sats: 7298, BTC: 7.298e-05}
+ *   POST {from_: "sat", to: "EUR",  amount: 7295}  -> {BTC: ..., sats: 7295, EUR: 5.4972}
+ *
+ * So a lookup of `data["sat"]` misses, which meant every sat-denominated
+ * conversion silently returned null.
+ */
+function pickConverted(data: Record<string, number>, to: string): number | undefined {
+  const lower = to.toLowerCase()
+  const candidates =
+    lower === 'sat' || lower === 'sats'
+      ? ['sats', 'sat']
+      : [to, to.toUpperCase(), lower]
+
+  for (const key of candidates) {
+    if (typeof data[key] === 'number') return data[key]
+  }
+  // Older/other shapes kept as a last resort.
+  return typeof data.amount === 'number'
+    ? data.amount
+    : typeof data.result === 'number'
+      ? data.result
+      : undefined
+}
+
 export function usePriceConversion() {
   const lnbitsAPI = injectService<LnbitsAPI>(SERVICE_TOKENS.LNBITS_API)
 
@@ -38,12 +69,7 @@ export function usePriceConversion() {
 
     try {
       const data = await lnbitsAPI.getConversion({ from, to, amount })
-      const result =
+      const result = pickConverted(data, to)
-        data[to] ??
-        data[to.toUpperCase()] ??
-        data[to.toLowerCase()] ??
-        (data as Record<string, number>).amount ??
-        (data as Record<string, number>).result
       if (typeof result !== 'number') return null
       cache.set(key, { value: result, expiresAt: Date.now() + TTL_MS })
       return result