Implement annotation syncing to and from server

lib/features/sheet_viewer/sheet_viewer_page.dart

@@ -6,6 +6,7 @@ import 'package:logging/logging.dart';
 import 'package:pdfrx/pdfrx.dart';
 import 'package:sheetless/core/models/config.dart';
 import 'package:sheetless/core/models/sheet.dart';
+import 'package:sheetless/core/services/annotation_sync_service.dart';
 import 'package:sheetless/core/services/api_client.dart';
 import 'package:sheetless/core/services/storage_service.dart';
 
@@ -35,6 +36,7 @@ class _SheetViewerPageState extends State<SheetViewerPage>
     with FullScreenListener {
   final _log = Logger('SheetViewerPage');
   final _storageService = StorageService();
+  late final AnnotationSyncService _syncService;
 
   PdfDocument? _document;
   late Future<bool> _documentLoaded;
@@ -52,6 +54,12 @@ class _SheetViewerPageState extends State<SheetViewerPage>
   void initState() {
     super.initState();
 
+    // Initialize sync service
+    _syncService = AnnotationSyncService(
+      apiClient: widget.apiClient,
+      storageService: _storageService,
+    );
+
     // Initialize drawing controllers
     _leftDrawingController = DrawingController(maxHistorySteps: 50);
     _rightDrawingController = DrawingController(maxHistorySteps: 50);
@@ -94,6 +102,9 @@ class _SheetViewerPageState extends State<SheetViewerPage>
       _totalPages = _document!.pages.length;
     });
 
+    // Sync annotations from server (downloads newer versions)
+    await _syncService.syncAnnotationsFromServer(widget.sheet.uuid);
+
     // Load annotations for current page(s)
     await _loadAnnotationsForCurrentPages();
 
@@ -131,24 +142,52 @@ class _SheetViewerPageState extends State<SheetViewerPage>
     }
   }
 
-  /// Saves the current page(s) annotations to storage.
+  /// Saves the current page(s) annotations to storage and uploads to server.
   Future<void> _saveCurrentAnnotations() async {
+    final now = DateTime.now();
+
     // Save left page
     final leftJson = _leftDrawingController.toJsonString();
-    await _storageService.writeAnnotations(
+    final leftHasContent = leftJson.isNotEmpty && leftJson != '[]';
+
+    await _storageService.writeAnnotationsWithMetadata(
       widget.sheet.uuid,
       _currentPage,
-      leftJson.isEmpty || leftJson == '[]' ? null : leftJson,
+      leftHasContent ? leftJson : null,
+      now,
     );
 
+    // Upload left page to server
+    if (leftHasContent) {
+      _syncService.uploadAnnotation(
+        sheetUuid: widget.sheet.uuid,
+        page: _currentPage,
+        annotationsJson: leftJson,
+        lastModified: now,
+      );
+    }
+
     // Save right page (two-page mode)
     if (widget.config.twoPageMode && _currentPage < _totalPages) {
       final rightJson = _rightDrawingController.toJsonString();
-      await _storageService.writeAnnotations(
+      final rightHasContent = rightJson.isNotEmpty && rightJson != '[]';
+
+      await _storageService.writeAnnotationsWithMetadata(
         widget.sheet.uuid,
         _currentPage + 1,
-        rightJson.isEmpty || rightJson == '[]' ? null : rightJson,
+        rightHasContent ? rightJson : null,
+        now,
       );
+
+      // Upload right page to server
+      if (rightHasContent) {
+        _syncService.uploadAnnotation(
+          sheetUuid: widget.sheet.uuid,
+          page: _currentPage + 1,
+          annotationsJson: rightJson,
+          lastModified: now,
+        );
+      }
     }
   }