Add background sync runner and rejected-sync UX

lib/features/sync/sync_view.dart

@@ -5,6 +5,7 @@ import 'package:relationship_saver/features/shared/frosted_card.dart';
 import 'package:relationship_saver/features/sync/sync_coordinator.dart';
 import 'package:relationship_saver/features/sync/sync_queue_repository.dart';
 import 'package:relationship_saver/features/sync/sync_state.dart';
+import 'package:relationship_saver/integrations/backend/models/backend_models.dart';
 
 class SyncView extends ConsumerStatefulWidget {
   const SyncView({super.key});
@@ -157,6 +158,39 @@ class _SyncViewState extends ConsumerState<SyncView> {
           ),
         ),
         const SizedBox(height: 16),
+        if (state.lastRejected.isNotEmpty) ...<Widget>[
+          FrostedCard(
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: <Widget>[
+                Wrap(
+                  spacing: 8,
+                  runSpacing: 8,
+                  crossAxisAlignment: WrapCrossAlignment.center,
+                  children: <Widget>[
+                    Text(
+                      'Rejected Changes',
+                      style: Theme.of(context).textTheme.titleMedium,
+                    ),
+                    TextButton.icon(
+                      onPressed: _busy ? null : _dismissRejections,
+                      icon: const Icon(Icons.done_all_rounded),
+                      label: const Text('Dismiss'),
+                    ),
+                  ],
+                ),
+                const SizedBox(height: 8),
+                ...state.lastRejected.map(
+                  (MutationRejection rejection) => Padding(
+                    padding: const EdgeInsets.only(bottom: 10),
+                    child: _rejectionTile(rejection),
+                  ),
+                ),
+              ],
+            ),
+          ),
+          const SizedBox(height: 16),
+        ],
         FrostedCard(
           child: Text(
             'Sync policy\n• Core usage never blocks on backend\n• Local changes queue first and remain safe offline\n• Pull applies backend envelopes into local state',
@@ -210,6 +244,53 @@ class _SyncViewState extends ConsumerState<SyncView> {
     );
   }
 
+  Widget _rejectionTile(MutationRejection rejection) {
+    return Container(
+      width: double.infinity,
+      padding: const EdgeInsets.all(12),
+      decoration: BoxDecoration(
+        color: Colors.white.withValues(alpha: 0.72),
+        borderRadius: BorderRadius.circular(12),
+      ),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: <Widget>[
+          Wrap(
+            spacing: 8,
+            runSpacing: 8,
+            crossAxisAlignment: WrapCrossAlignment.center,
+            children: <Widget>[
+              Container(
+                padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 3),
+                decoration: BoxDecoration(
+                  color: const Color(0xFFEAF7FB),
+                  borderRadius: BorderRadius.circular(999),
+                ),
+                child: Text(
+                  rejection.code,
+                  style: Theme.of(
+                    context,
+                  ).textTheme.labelMedium?.copyWith(color: AppTheme.primary),
+                ),
+              ),
+              Text(
+                'Mutation ${rejection.clientMutationId}',
+                style: Theme.of(
+                  context,
+                ).textTheme.bodySmall?.copyWith(color: AppTheme.textSecondary),
+              ),
+            ],
+          ),
+          const SizedBox(height: 6),
+          Text(
+            rejection.message,
+            style: Theme.of(context).textTheme.bodyMedium,
+          ),
+        ],
+      ),
+    );
+  }
+
   String _formatDateTime(DateTime? value) {
     if (value == null) {
       return 'never';
@@ -254,6 +335,25 @@ class _SyncViewState extends ConsumerState<SyncView> {
     }
   }
 
+  Future<void> _dismissRejections() async {
+    setState(() {
+      _busy = true;
+    });
+
+    try {
+      await ref.read(syncQueueRepositoryProvider.notifier).clearRejections();
+      setState(() {
+        _status = 'Dismissed latest rejected-mutation details.';
+      });
+    } finally {
+      if (mounted) {
+        setState(() {
+          _busy = false;
+        });
+      }
+    }
+  }
+
   Future<void> _run(
     Future<SyncRunResult> Function(SyncCoordinator coordinator) action,
   ) async {