- UndoableAction interface with AddStrokeAction and DeleteStrokeAction
- UndoManager: undo/redo stacks (depth 50), canUndo/canRedo StateFlows
- EditorViewModel: stroke operations routed through UndoManager,
visual callbacks sync canvas view without full DB reload
- Toolbar: undo/redo buttons with enabled state
- 9 unit tests for UndoManager
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
