Only save annotations on changes

lib/features/sheet_viewer/drawing/drawing_controller.dart

@@ -51,6 +51,9 @@ class DrawingController extends ChangeNotifier {
   /// Maximum number of history steps to keep
   final int maxHistorySteps;
 
+  /// Whether there are unsaved changes since last load/clear
+  bool _hasUnsavedChanges = false;
+
   DrawingController({this.maxHistorySteps = 50});
 
   // ---------------------------------------------------------------------------
@@ -75,6 +78,9 @@ class DrawingController extends ChangeNotifier {
   /// Whether redo is available
   bool get canRedo => _redoStack.isNotEmpty;
 
+  /// Whether there are unsaved changes since last load/clear/markSaved
+  bool get hasUnsavedChanges => _hasUnsavedChanges;
+
   // ---------------------------------------------------------------------------
   // Drawing Operations
   // ---------------------------------------------------------------------------
@@ -120,6 +126,7 @@ class DrawingController extends ChangeNotifier {
         _redoStack.clear();
         _trimHistory();
         _currentErasedLines.clear();
+        _hasUnsavedChanges = true;
         notifyListeners(); // Update UI to enable undo button
       }
       return;
@@ -133,6 +140,7 @@ class DrawingController extends ChangeNotifier {
       _undoStack.add(AddLineAction(_currentLine!));
       _redoStack.clear();
       _trimHistory();
+      _hasUnsavedChanges = true;
     }
 
     _currentLine = null;
@@ -257,6 +265,7 @@ class DrawingController extends ChangeNotifier {
         _redoStack.add(action);
     }
 
+    _hasUnsavedChanges = true;
     notifyListeners();
   }
 
@@ -279,6 +288,7 @@ class DrawingController extends ChangeNotifier {
         _undoStack.add(action);
     }
 
+    _hasUnsavedChanges = true;
     notifyListeners();
   }
 
@@ -289,6 +299,7 @@ class DrawingController extends ChangeNotifier {
     _redoStack.clear();
     _currentLine = null;
     _currentErasedLines.clear();
+    _hasUnsavedChanges = false;
     notifyListeners();
   }
 
@@ -324,6 +335,7 @@ class DrawingController extends ChangeNotifier {
     _redoStack.clear();
     _currentLine = null;
     _currentErasedLines.clear();
+    _hasUnsavedChanges = false;
 
     for (final json in jsonList) {
       _lines.add(DrawingLine.fromJson(json));
@@ -354,6 +366,11 @@ class DrawingController extends ChangeNotifier {
     notifyListeners();
   }
 
+  /// Marks the current state as saved (resets unsaved changes flag).
+  void markSaved() {
+    _hasUnsavedChanges = false;
+  }
+
   @override
   void dispose() {
     _lines.clear();