Added Google Drive storage location

lib/services/merge_utils.dart

@@ -0,0 +1,23 @@
+/// Unions [remote] and [local] by ID, keeping whichever copy of each
+/// record has the newer `updatedAt` when a record exists on both sides.
+/// This lets two devices' independent new records both survive a sync;
+/// it does not propagate deletions (see plan doc's Known limitations).
+List<T> mergeById<T>(
+  List<T> remote,
+  List<T> local,
+  String Function(T) idOf,
+  DateTime Function(T) updatedAtOf,
+) {
+  final merged = <String, T>{};
+  for (final item in remote) {
+    merged[idOf(item)] = item;
+  }
+  for (final item in local) {
+    final id = idOf(item);
+    final existing = merged[id];
+    if (existing == null || !updatedAtOf(existing).isAfter(updatedAtOf(item))) {
+      merged[id] = item;
+    }
+  }
+  return merged.values.toList();
+}