Implement annotation syncing to and from server

lib/core/services/api_client.dart

@@ -183,6 +183,61 @@ class ApiClient {
     _log.info('PDF cached at: ${cachedFile.path}');
     return cachedFile;
   }
+
+  // ---------------------------------------------------------------------------
+  // Annotation Operations
+  // ---------------------------------------------------------------------------
+
+  /// Fetches all annotations for a sheet from the server.
+  ///
+  /// Returns a list of [ServerAnnotation] objects containing page number,
+  /// lastModified timestamp, and the annotations JSON string.
+  Future<List<ServerAnnotation>> fetchAnnotations(String sheetUuid) async {
+    final response = await get('/api/sheets/$sheetUuid/annotations');
+    final data = jsonDecode(response.body) as List<dynamic>;
+
+    return data
+        .map((item) => ServerAnnotation.fromJson(item as Map<String, dynamic>))
+        .toList();
+  }
+
+  /// Uploads annotations for a specific page of a sheet.
+  ///
+  /// The [lastModified] should be the current time when the annotation was saved.
+  Future<void> uploadAnnotation({
+    required String sheetUuid,
+    required int page,
+    required DateTime lastModified,
+    required String annotationsJson,
+  }) async {
+    await post('/api/sheets/$sheetUuid/annotations', {
+      'page': page,
+      'lastModified': lastModified.toIso8601String(),
+      'annotations': annotationsJson,
+    });
+    _log.info('Annotation uploaded for sheet $sheetUuid page $page');
+  }
+}
+
+/// Represents an annotation from the server.
+class ServerAnnotation {
+  final int page;
+  final DateTime lastModified;
+  final String annotationsJson;
+
+  ServerAnnotation({
+    required this.page,
+    required this.lastModified,
+    required this.annotationsJson,
+  });
+
+  factory ServerAnnotation.fromJson(Map<String, dynamic> json) {
+    return ServerAnnotation(
+      page: json['page'] as int,
+      lastModified: DateTime.parse(json['lastModified'] as String),
+      annotationsJson: json['annotations'] as String,
+    );
+  }
 }
 
 /// Exception thrown when an API request fails.