Added Google Drive storage location

lib/services/lock_coordinator.dart

@@ -0,0 +1,48 @@
+import 'drive_service.dart' show DriveLockFile;
+
+/// Ticket-based mutex using timestamped lock files as the coordination
+/// point: create a lock named after our own timestamp, then wait until no
+/// *other* lock older than ours remains — reaping ("deleting") any it
+/// finds older than [staleAge] along the way, as a backstop against a
+/// device that crashed/went offline before releasing its own lock.
+///
+/// Pure orchestration over injected operations (rather than a concrete
+/// Drive dependency) so the state machine is unit-testable without a real
+/// network connection.
+Future<String> acquireLock({
+  required String username,
+  required Future<String> Function(String lockName) createLock,
+  required Future<List<DriveLockFile>> Function() listLocks,
+  required Future<void> Function(String lockId) deleteLock,
+  DateTime Function()? nowUtc,
+  Future<void> Function(Duration)? delay,
+  Duration staleAge = const Duration(minutes: 10),
+  Duration pollInterval = const Duration(seconds: 1),
+}) async {
+  final now = nowUtc ?? () => DateTime.now().toUtc();
+  final wait = delay ?? Future.delayed;
+
+  final epochMillis = now().millisecondsSinceEpoch;
+  final lockName = '$username-$epochMillis.lock';
+  final lockId = await createLock(lockName);
+
+  while (true) {
+    final locks = await listLocks();
+    final others = locks.where((l) => l.id != lockId);
+    final olderOthers =
+        others.where((l) => l.createdAtUtc.millisecondsSinceEpoch < epochMillis).toList();
+
+    if (olderOthers.isEmpty) break;
+
+    final current = now();
+    for (final stale in olderOthers) {
+      if (current.difference(stale.createdAtUtc) > staleAge) {
+        await deleteLock(stale.id);
+      }
+    }
+
+    await wait(pollInterval);
+  }
+
+  return lockId;
+}