Implement offline mode

lib/core/services/annotation_sync_service.dart

@@ -15,8 +15,8 @@ class AnnotationSyncService {
   AnnotationSyncService({
     required ApiClient apiClient,
     required StorageService storageService,
-  }) : _apiClient = apiClient,
-       _storageService = storageService;
+  })  : _apiClient = apiClient,
+        _storageService = storageService;
 
   /// Downloads annotations from server and merges with local storage.
   ///
@@ -31,8 +31,8 @@ class AnnotationSyncService {
       final serverAnnotations = await _apiClient.fetchAnnotations(sheetUuid);
 
       // Get all local annotations with metadata
-      final localAnnotations = await _storageService
-          .readAllAnnotationsWithMetadata(sheetUuid);
+      final localAnnotations =
+          await _storageService.readAllAnnotationsWithMetadata(sheetUuid);
 
       int updatedCount = 0;
 
@@ -88,7 +88,7 @@ class AnnotationSyncService {
   /// Uploads a single page's annotation to the server.
   ///
   /// Called when annotations are saved (e.g., exiting paint mode).
-  /// Silently fails if upload fails (allows offline usage).
+  /// If upload fails (e.g., offline), the annotation is queued for later sync.
   Future<bool> uploadAnnotation({
     required String sheetUuid,
     required int page,
@@ -108,11 +108,42 @@ class AnnotationSyncService {
       _log.info('Upload successful');
       return true;
     } on ApiException catch (e) {
-      _log.warning('Failed to upload annotation: $e');
+      _log.warning('Failed to upload annotation, queuing for later: $e');
+      await _queueForLaterUpload(
+        sheetUuid: sheetUuid,
+        page: page,
+        annotationsJson: annotationsJson,
+        lastModified: lastModified,
+      );
       return false;
     } catch (e) {
-      _log.warning('Unexpected error uploading annotation: $e');
+      _log.warning(
+          'Unexpected error uploading annotation, queuing for later: $e');
+      await _queueForLaterUpload(
+        sheetUuid: sheetUuid,
+        page: page,
+        annotationsJson: annotationsJson,
+        lastModified: lastModified,
+      );
       return false;
     }
   }
+
+  /// Queues an annotation for later upload when connection is restored.
+  Future<void> _queueForLaterUpload({
+    required String sheetUuid,
+    required int page,
+    required String annotationsJson,
+    required DateTime lastModified,
+  }) async {
+    await _storageService.writePendingAnnotationUpload(
+      PendingAnnotationUpload(
+        sheetUuid: sheetUuid,
+        page: page,
+        annotationsJson: annotationsJson,
+        lastModified: lastModified,
+      ),
+    );
+    _log.info('Annotation queued for later upload: $sheetUuid page $page');
+  }
 }