feat: implement comprehensive canvas context menu system

Adds right-click context menu for canvas with full operation support:

**Clipboard Operations:**
- Cut/Copy/Paste with selection mask support
- Browser clipboard API integration for external images
- Internal clipboard buffer for canvas selections
- Toast notifications for user feedback

**Selection Operations:**
- Select All - creates full canvas selection with proper mask
- Deselect - clears active selection
- Selection state properly integrated with canvas operations

**Layer Operations:**
- New Layer - creates layer with history support
- Duplicate Layer - clones active layer
- Merge Down - merges layer with one below

**Transform Operations:**
- Rotate 90° CW - rotates active layer clockwise
- Flip Horizontal - mirrors layer horizontally
- Flip Vertical - mirrors layer vertically
- All transforms preserve image quality and support undo/redo

**Edit Operations:**
- Undo/Redo - integrated with history system
- Disabled states for unavailable operations
- Context-aware menu items

**New Files Created:**
- lib/clipboard-operations.ts - Cut/copy/paste implementation
- lib/canvas-operations.ts - Rotate/flip canvas functions

**Modified Files:**
- components/canvas/canvas-with-tools.tsx - Context menu integration
- store/selection-store.ts - Added selectAll() method
- core/commands/index.ts - Export all command types

**Technical Improvements:**
- Proper Selection type structure with mask/bounds
- History command integration for all operations
- Lazy-loaded operations for performance
- Toast feedback for all user actions
- Full TypeScript type safety

All operations work with undo/redo and maintain app state consistency.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/commands/index.ts

@@ -2,3 +2,6 @@ export * from './base-command';
 export * from './layer-commands';
 export * from './draw-command';
 export * from './move-command';
+export * from './transform-command';
+export * from './filter-command';
+export * from './selection-command';