Add background sync runner and rejected-sync UX

lib/features/sync/sync_queue_repository.dart

@@ -127,6 +127,22 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
     );
   }
 
+  /// Clears the latest rejection payload shown to the user.
+  Future<void> clearRejections() async {
+    final SyncState current = await _currentState();
+    final String? lastError =
+        current.lastError != null &&
+            current.lastError!.startsWith('Push rejected')
+        ? null
+        : current.lastError;
+    await _setState(
+      current.copyWith(
+        lastRejected: const <MutationRejection>[],
+        lastError: lastError,
+      ),
+    );
+  }
+
   Future<SyncState> _currentState() async {
     final SyncState? value = state.asData?.value;
     if (value != null) {