feat(sync): offer-to-pull encrypted settings on login (one-time, non-blocking)

Completes todo #15. On login, if an encrypted copy of the user's storage and/or
wallet settings exists on their Nostr account — and this device hasn't already
opted in — offer to restore it.

- js/nostr/encryptedSync.js: exists(dTag) — cheap list-only existence peek (NO
  decrypt, so it doesn't spend the signer budget); decryption only on accept.
- storage/walletSyncService: hasRemote() wrappers over exists().
- js/services/settingsRestorePrompt.js: one-time per-pubkey offer. Only prompts
  for items with a remote copy not already enabled locally; if nothing remote
  exists it does NOT consume the one-time offer (a note published later can still
  prompt next login). On accept, pulls + applies.
- app.js handleLoginModalSuccess: fires maybeOfferSettingsRestore(pubkey) after
  login, deferred ~2s so relays connect; best-effort, never blocks login; toasts
  on restore.

tests/settings-restore-prompt.test.mjs: offer+restore-only-remote-items, decline,
prompt-once, skip-already-enabled, don't-consume-when-empty. Mutation-verified.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/services/storageSyncService.js

@@ -35,6 +35,16 @@ export function createStorageSyncService({
     setSyncEnabled(normalizePubkey(pubkey), SYNC_KIND, enabled);
   }
 
+  // Cheap check (no decrypt) for whether a synced note exists — for the
+  // login-restore prompt.
+  async function hasRemote() {
+    if (typeof encryptedSync.exists !== "function") {
+      return false;
+    }
+    const result = await encryptedSync.exists(STORAGE_SYNC_DTAG);
+    return Boolean(result?.exists);
+  }
+
   // Push the current local storage account record to the user's encrypted note.
   async function push(pubkey) {
     const key = normalizePubkey(pubkey);
@@ -90,6 +100,7 @@ export function createStorageSyncService({
     STORAGE_SYNC_DTAG,
     isAvailable,
     isEnabled,
+    hasRemote,
     push,
     pull,
     enable,