Add rejected mutation requeue flow in sync UI

lib/features/sync/sync_queue_repository.dart

@@ -68,22 +68,39 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
   Future<void> applyPushResult(SyncPushResult result, {DateTime? at}) async {
     final SyncState current = await _currentState();
     final DateTime now = at ?? DateTime.now();
-    final Set<String> completedMutationIds = <String>{
-      ...result.accepted.map((MutationAck ack) => ack.clientMutationId),
+    final Set<String> rejectedMutationIds = <String>{
       ...result.rejected.map(
         (MutationRejection rejection) => rejection.clientMutationId,
       ),
     };
+    final Set<String> completedMutationIds = <String>{
+      ...result.accepted.map((MutationAck ack) => ack.clientMutationId),
+      ...rejectedMutationIds,
+    };
+    final List<ChangeEnvelope> rejectedChanges = current.pendingChanges
+        .where(
+          (ChangeEnvelope change) =>
+              rejectedMutationIds.contains(change.clientMutationId),
+        )
+        .toList(growable: false);
     final List<ChangeEnvelope> pending = current.pendingChanges
         .where(
           (ChangeEnvelope change) =>
               !completedMutationIds.contains(change.clientMutationId),
         )
         .toList(growable: false);
+    final List<ChangeEnvelope> dedupedRejectedChanges = rejectedChanges
+        .where(
+          (ChangeEnvelope change) => !pending.any(
+            (ChangeEnvelope pendingChange) =>
+                pendingChange.clientMutationId == change.clientMutationId,
+          ),
+        )
+        .toList(growable: false);
 
     final String? rejectionMessage = result.rejected.isEmpty
         ? null
-        : 'Push rejected ${result.rejected.length} change(s).';
+        : 'Push rejected ${result.rejected.length} change(s). Fix locally and retry.';
 
     await _setState(
       current.copyWith(
@@ -93,6 +110,29 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
         lastSyncAt: now,
         lastError: rejectionMessage,
         lastRejected: result.rejected,
+        lastRejectedChanges: dedupedRejectedChanges,
+      ),
+    );
+  }
+
+  /// Requeues rejected changes to pending sync list for manual retry.
+  Future<void> requeueRejectedChanges() async {
+    final SyncState current = await _currentState();
+    if (current.lastRejectedChanges.isEmpty) {
+      return;
+    }
+
+    final List<ChangeEnvelope> pending = <ChangeEnvelope>[
+      ...current.lastRejectedChanges,
+      ...current.pendingChanges,
+    ];
+    final List<ChangeEnvelope> dedupedPending = _dedupeByMutationId(pending);
+    await _setState(
+      current.copyWith(
+        pendingChanges: dedupedPending,
+        lastRejected: const <MutationRejection>[],
+        lastRejectedChanges: const <ChangeEnvelope>[],
+        lastError: null,
       ),
     );
   }
@@ -107,6 +147,7 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
         lastSyncAt: now,
         lastError: null,
         lastRejected: const <MutationRejection>[],
+        lastRejectedChanges: const <ChangeEnvelope>[],
       ),
     );
   }
@@ -123,6 +164,7 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
       current.copyWith(
         pendingChanges: const <ChangeEnvelope>[],
         lastRejected: const <MutationRejection>[],
+        lastRejectedChanges: const <ChangeEnvelope>[],
       ),
     );
   }
@@ -138,11 +180,23 @@ class SyncQueueRepository extends AsyncNotifier<SyncState> {
     await _setState(
       current.copyWith(
         lastRejected: const <MutationRejection>[],
+        lastRejectedChanges: const <ChangeEnvelope>[],
         lastError: lastError,
       ),
     );
   }
 
+  List<ChangeEnvelope> _dedupeByMutationId(List<ChangeEnvelope> changes) {
+    final Set<String> seen = <String>{};
+    final List<ChangeEnvelope> deduped = <ChangeEnvelope>[];
+    for (final ChangeEnvelope change in changes) {
+      if (seen.add(change.clientMutationId)) {
+        deduped.add(change);
+      }
+    }
+    return deduped;
+  }
+
   Future<SyncState> _currentState() async {
     final SyncState? value = state.asData?.value;
     if (value != null) {