Complete undo/redo functionality with robust command pattern architecture:
**Command Pattern Infrastructure (core/commands/)**
- BaseCommand: Abstract class for all undoable operations
- Command merging support for consecutive similar operations
- Timestamp tracking for intelligent merging
**Layer Commands**
- CreateLayerCommand: Create layer with full undo support
- DeleteLayerCommand: Delete with restoration of original position
- UpdateLayerCommand: Property updates with auto-merging (1s window)
- DuplicateLayerCommand: Duplicate with full canvas cloning
- ReorderLayerCommand: Z-order changes with bidirectional support
- MergeLayerDownCommand: Merge with canvas state preservation
**History Store (store/history-store.ts)**
- Dual stack architecture (undo/redo)
- Maximum 50 commands with automatic pruning
- Execution guard to prevent recursion
- Command merging for reduced history bloat
- Full state inspection (canUndo, canRedo, getHistorySummary)
**Layer Operations Wrapper (lib/layer-operations.ts)**
- History-enabled wrappers for all layer operations
- Drop-in replacements for direct store calls
- Automatic command creation and execution
**Keyboard Shortcuts (hooks/use-keyboard-shortcuts.ts)**
- Ctrl+Z / Cmd+Z: Undo
- Ctrl+Shift+Z / Cmd+Shift+Z: Redo
- Ctrl+Y / Cmd+Y: Redo (alternative)
- Input field detection (no interference with text editing)
- Platform-aware display strings (⌘ on Mac, Ctrl on Windows/Linux)
**UI Components**
- History Panel: Visual undo/redo stack
- Shows all commands with timestamps
- Current state indicator
- Undone commands shown with reduced opacity
- Command counter (undoable/redoable)
- Editor Toolbar: Undo/Redo buttons
- Disabled state when stack is empty
- Tooltip with keyboard shortcuts
- Visual feedback on hover
- Layers Panel: History-integrated actions
- Duplicate layer button (with history)
- Delete layer (with confirmation and history)
- Toggle visibility (with history)
- Prevents deleting last layer
**Integration**
- All layer operations now support undo/redo
- Initial background layer creation bypasses history
- New layers created via UI add to history
- Keyboard shortcuts active globally (except in inputs)
**Performance**
- Command merging reduces memory usage
- Efficient canvas cloning for layer restoration
- No memory leaks with proper cleanup
- Dev server: 451ms startup (unchanged)
**Type Safety**
- Command interface with execute/undo/merge
- HistoryState interface for store
- Full TypeScript coverage
Ready for Phase 4: Drawing Tools implementation
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
