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>

store/selection-store.ts

@@ -14,6 +14,7 @@ interface SelectionStore extends SelectionState {
   setTolerance: (tolerance: number) => void;
   setMarching: (isMarching: boolean) => void;
   clearSelection: () => void;
+  selectAll: () => void;
   invertSelection: () => void;
 }
 
@@ -60,6 +61,40 @@ export const useSelectionStore = create<SelectionStore>((set) => ({
       activeSelection: null,
     }),
 
+  selectAll: () =>
+    set(() => {
+      const { useCanvasStore, useLayerStore } = require('@/store');
+      const { width, height } = useCanvasStore.getState();
+      const { getActiveLayer } = useLayerStore.getState();
+      const activeLayer = getActiveLayer();
+
+      if (!activeLayer) return {};
+
+      // Create a mask that covers the entire canvas
+      const maskData = new Uint8Array(width * height).fill(255);
+
+      return {
+        activeSelection: {
+          id: `selection-${Date.now()}`,
+          layerId: activeLayer.id,
+          mask: {
+            width,
+            height,
+            data: maskData,
+            bounds: {
+              x: 0,
+              y: 0,
+              width,
+              height,
+            },
+          },
+          inverted: false,
+          feather: 0,
+          createdAt: Date.now(),
+        },
+      };
+    }),
+
   invertSelection: () =>
     set((state) => {
       if (state.activeSelection) {