fix(lite): flush the wallet save on quit before _Exit

On quit the lite controller's destructor never runs, because main()
ends the process with _Exit(). Any state the background refresh had
scanned since the last save was lost: the destructor is where that
final save would normally happen, and it was being skipped on the
one path that always runs.

- shutdown() now calls a new flushOnExit() that persists once on the
  way out, bounded to 4 s so the existing 15 s shutdown watchdog
  budget still holds. On timeout it abandons the save thread and lets
  _Exit follow.
- It is a plain save through the same backend path, which writes a
  temp file and renames it into place. That cannot corrupt the wallet
  if it is cut off or abandoned, and it does not change the wallet
  file format - it only persists sooner on the quit path.
- It reuses the destructor's guards and also skips when a wallet op
  is already in progress: shutdown() has already waited for ops to go
  idle, and a still-pending op holds the sync lock and issues its own
  save, so flushOnExit would only block the budget on a save that
  cannot take the lock.

Tests cover a save on the open, synced, idle path (exactly once), a
blocked save returning within the budget, and the retired and
never-opened guards skipping the save.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.cpp

@@ -7742,6 +7742,9 @@ void App::shutdown()
     // Lite: let an in-flight wallet op (e.g. Encrypt's save) finish. Bounded: the window is already hidden and
     // main.cpp has a 15 s watchdog. The backend writes tmp + rename, so being cut off never corrupts the wallet.
     if (lite_wallet_) lite_wallet_->waitWalletOpsIdle(std::chrono::milliseconds(2500));
+    // Persist refresh-scanned state: ~LiteWalletController never runs on the _Exit path. Bounded (4 s) so the
+    // 15 s watchdog budget holds; the save is tmp + rename, so an abandoned/cut-off save never corrupts.
+    if (lite_wallet_) lite_wallet_->flushOnExit(std::chrono::milliseconds(4000));
 
     // Full node: persist whatever a refresh changed since beginShutdown() (everything, on the direct-exit path),
     // then drain + join the history writer. Bounded like the settings flush: on a wedged disk the in-flight write