fix(storage): ensure single-flight thread-safe LocalDb._open initialization

[ZhuchkaTriplesix]

Motivation & Problem

In lib/core/storage/local_db.dart, _open() currently performs:

Future<Database> _open() async {
  if (_db != null && _db!.isOpen) return _db!;
  await initFfi();
  ...
  _db = await databaseFactoryFfi.openDatabase(...);
  return _db!;
}

When multiple operations query LocalDb simultaneously on cold startup (e.g. AppSettings.getTheme(), LocalDb.listConnections(), and LocalDb.listFolders()), all parallel callers can enter _open() before the first openDatabase() finishes. This can lead to multiple concurrent SQLite open attempts on the same file path before _db is assigned.

Proposed Scope

1. In LocalDb:
   • Introduce a single-flight memoization mechanism (e.g. Future<Database>? _openFuture) to guarantee that concurrent callers share the exact same opening Future.
   • Clear _openFuture on close() or if database opening throws an error.
2. Add unit tests in local_db_test.dart verifying that concurrent simultaneous calls to _open() / database operations safely complete with a single open handle.

Acceptance Criteria

• Parallel concurrent invocations of LocalDb._open() are safely deduplicated (single-flight).
• Database handle lifecycle is cleanly reset on LocalDb.instance.close().
• All database tests pass reliably.