fix: normalize storage lock failures

src/lnurlcash/storageLock.ts

@@ -3,12 +3,13 @@
 // lose each other's records (worst case: a stale tab overwrites a freshly
 // persisted rotated note after its old k1 was burned). Falls back to
 // running unlocked where Web Locks is unavailable (plain-Node tests, very
-// old browsers).
-export const withStorageLock = <T>(
-  name: string,
-  fn: () => T | Promise<T>
-): Promise<T> => {
+// old browsers). That fallback provides no cross-tab serialization
+// guarantee; the promise hop only normalizes synchronous callback errors.
+export const withStorageLock = <T>(name: string, fn: () => T | Promise<T>): Promise<T> => {
   const locks = typeof navigator !== 'undefined' ? navigator.locks : undefined
-  if (locks) return locks.request(name, fn)
-  return Promise.resolve(fn())
+  if (locks) return locks.request(name, () => Promise.resolve().then(fn))
+  return Promise.resolve().then(fn)
 }
+
+export const storageLocksAvailable = (): boolean =>
+  typeof navigator !== 'undefined' && navigator.locks !== undefined