feat(phase-13): implement canvas resize dialog with multiple methods

Add comprehensive canvas resize functionality with three resize methods.

Features:
- Canvas Size dialog accessible from Image menu
- Three resize methods:
  * Scale: Stretch/shrink content to fit new size
  * Crop: Crop or center content without scaling
  * Expand: Expand canvas, center existing content
- Maintain aspect ratio toggle with link/unlink button
- Dimension validation (1-10000 pixels)
- Real-time preview of new dimensions
- Applies to all layers simultaneously
- Toast notifications for success/errors

Changes:
- Created components/modals/canvas-resize-dialog.tsx
- Added "Canvas Size..." menu item to Image menu
- Integrated with useLayerStore for layer updates
- Uses toast utility for user feedback
- Validates dimensions before applying
- Supports linked/unlinked aspect ratio

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

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

components/editor/image-menu.tsx

@@ -2,14 +2,17 @@
 
 import { useState } from 'react';
 import { AdjustmentsDialog } from '@/components/modals/adjustments-dialog';
+import { CanvasResizeDialog } from '@/components/modals/canvas-resize-dialog';
 import {
   Sliders,
   ChevronDown,
+  Maximize2,
 } from 'lucide-react';
 
 export function ImageMenu() {
   const [isMenuOpen, setIsMenuOpen] = useState(false);
   const [isAdjustmentsOpen, setIsAdjustmentsOpen] = useState(false);
+  const [isCanvasResizeOpen, setIsCanvasResizeOpen] = useState(false);
 
   return (
     <>
@@ -48,6 +51,17 @@ export function ImageMenu() {
                 <Sliders className="h-4 w-4" aria-hidden="true" />
                 Filters & Adjustments...
               </button>
+              <button
+                onClick={() => {
+                  setIsCanvasResizeOpen(true);
+                  setIsMenuOpen(false);
+                }}
+                className="flex items-center gap-3 w-full px-4 py-2 text-sm hover:bg-accent transition-colors focus:outline-none focus:bg-accent text-left"
+                role="menuitem"
+              >
+                <Maximize2 className="h-4 w-4" aria-hidden="true" />
+                Canvas Size...
+              </button>
             </div>
           </>
         )}
@@ -58,6 +72,12 @@ export function ImageMenu() {
         isOpen={isAdjustmentsOpen}
         onClose={() => setIsAdjustmentsOpen(false)}
       />
+
+      {/* Canvas Resize Dialog */}
+      <CanvasResizeDialog
+        isOpen={isCanvasResizeOpen}
+        onClose={() => setIsCanvasResizeOpen(false)}
+      />
     </>
   );
 }