Implement Phase 6: undo/redo with command pattern

- 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>

PROGRESS.md

@@ -71,9 +71,18 @@ See PROJECT_PLAN.md for the full step list.
   processes historical touch points for thorough erasing, deletes from
   view + Room DB, rebuilds backing bitmap
 
+### Phase 6: Undo/Redo (2026-03-24)
+
+- [x] 6.1: UndoableAction interface, AddStrokeAction, DeleteStrokeAction
+- [x] 6.2: UndoManager with undo/redo stacks (depth 50), StateFlow for canUndo/canRedo
+- [x] 6.3: EditorViewModel rewired — stroke add/delete go through UndoManager,
+  visual callbacks sync canvas view without full reload
+- [x] 6.4: 9 unit tests for UndoManager (perform, undo, redo, depth limit, clear, no-ops)
+- Toolbar now has undo/redo buttons with enabled state
+
 ## In Progress
 
-Phase 6: Undo/Redo
+Phase 7: Selection
 
 ## Decisions & Deviations