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)}
+      />
     </>
   );
 }

components/modals/canvas-resize-dialog.tsx

@@ -0,0 +1,238 @@
+'use client';
+
+import { useState, useCallback } from 'react';
+import { X, Link, Unlink } from 'lucide-react';
+import { useLayerStore, useCanvasStore } from '@/store';
+import { toast } from '@/lib/toast-utils';
+
+interface CanvasResizeDialogProps {
+  isOpen: boolean;
+  onClose: () => void;
+}
+
+export function CanvasResizeDialog({ isOpen, onClose }: CanvasResizeDialogProps) {
+  const { layers } = useLayerStore();
+
+  // Get current canvas dimensions from first layer
+  const currentWidth = layers[0]?.width || 800;
+  const currentHeight = layers[0]?.height || 600;
+
+  const [width, setWidth] = useState(currentWidth);
+  const [height, setHeight] = useState(currentHeight);
+  const [maintainAspectRatio, setMaintainAspectRatio] = useState(true);
+  const [resizeMethod, setResizeMethod] = useState<'scale' | 'crop' | 'expand'>('scale');
+
+  const aspectRatio = currentWidth / currentHeight;
+
+  const handleWidthChange = useCallback(
+    (newWidth: number) => {
+      setWidth(newWidth);
+      if (maintainAspectRatio) {
+        setHeight(Math.round(newWidth / aspectRatio));
+      }
+    },
+    [maintainAspectRatio, aspectRatio]
+  );
+
+  const handleHeightChange = useCallback(
+    (newHeight: number) => {
+      setHeight(newHeight);
+      if (maintainAspectRatio) {
+        setWidth(Math.round(newHeight * aspectRatio));
+      }
+    },
+    [maintainAspectRatio, aspectRatio]
+  );
+
+  const handleApply = useCallback(() => {
+    if (width < 1 || height < 1 || width > 10000 || height > 10000) {
+      toast.error('Width and height must be between 1 and 10000 pixels');
+      return;
+    }
+
+    // Apply resize based on method
+    const { updateLayer } = useLayerStore.getState();
+
+    layers.forEach((layer) => {
+      if (!layer.canvas) return;
+
+      const newCanvas = document.createElement('canvas');
+      newCanvas.width = width;
+      newCanvas.height = height;
+      const ctx = newCanvas.getContext('2d');
+      if (!ctx) return;
+
+      switch (resizeMethod) {
+        case 'scale':
+          // Scale the content to fit new size
+          ctx.drawImage(layer.canvas, 0, 0, width, height);
+          break;
+        case 'crop':
+          // Crop/center the content
+          const offsetX = Math.max(0, (width - layer.width) / 2);
+          const offsetY = Math.max(0, (height - layer.height) / 2);
+          ctx.drawImage(layer.canvas, offsetX, offsetY);
+          break;
+        case 'expand':
+          // Expand canvas, center existing content
+          const centerX = (width - layer.width) / 2;
+          const centerY = (height - layer.height) / 2;
+          ctx.drawImage(layer.canvas, centerX, centerY);
+          break;
+      }
+
+      updateLayer(layer.id, {
+        canvas: newCanvas,
+        width,
+        height,
+      });
+    });
+
+    toast.success(`Canvas resized to ${width} × ${height}px`);
+
+    onClose();
+  }, [width, height, resizeMethod, layers, onClose]);
+
+  if (!isOpen) return null;
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/50">
+      <div className="bg-card border border-border rounded-lg shadow-lg w-full max-w-md">
+        {/* Header */}
+        <div className="flex items-center justify-between border-b border-border p-4">
+          <h2 className="text-lg font-semibold text-foreground">Canvas Size</h2>
+          <button
+            onClick={onClose}
+            className="text-muted-foreground hover:text-foreground transition-colors"
+          >
+            <X className="h-5 w-5" />
+          </button>
+        </div>
+
+        {/* Content */}
+        <div className="p-4 space-y-4">
+          {/* Current Size */}
+          <div>
+            <p className="text-sm text-muted-foreground">
+              Current size: {currentWidth} × {currentHeight}px
+            </p>
+          </div>
+
+          {/* New Dimensions */}
+          <div className="space-y-3">
+            <div className="flex items-center gap-3">
+              <div className="flex-1">
+                <label className="text-sm font-medium text-foreground">Width (px)</label>
+                <input
+                  type="number"
+                  min="1"
+                  max="10000"
+                  value={width}
+                  onChange={(e) => handleWidthChange(Number(e.target.value))}
+                  className="w-full mt-1 px-3 py-2 border border-border rounded-md bg-background text-foreground"
+                />
+              </div>
+
+              <button
+                onClick={() => setMaintainAspectRatio(!maintainAspectRatio)}
+                className="mt-6 p-2 text-muted-foreground hover:text-foreground transition-colors"
+                title={maintainAspectRatio ? 'Unlink aspect ratio' : 'Link aspect ratio'}
+              >
+                {maintainAspectRatio ? (
+                  <Link className="h-4 w-4" />
+                ) : (
+                  <Unlink className="h-4 w-4" />
+                )}
+              </button>
+
+              <div className="flex-1">
+                <label className="text-sm font-medium text-foreground">Height (px)</label>
+                <input
+                  type="number"
+                  min="1"
+                  max="10000"
+                  value={height}
+                  onChange={(e) => handleHeightChange(Number(e.target.value))}
+                  className="w-full mt-1 px-3 py-2 border border-border rounded-md bg-background text-foreground"
+                />
+              </div>
+            </div>
+          </div>
+
+          {/* Resize Method */}
+          <div>
+            <label className="text-sm font-medium text-foreground">Resize Method</label>
+            <div className="mt-2 space-y-2">
+              <label className="flex items-center gap-2 cursor-pointer">
+                <input
+                  type="radio"
+                  name="resizeMethod"
+                  value="scale"
+                  checked={resizeMethod === 'scale'}
+                  onChange={() => setResizeMethod('scale')}
+                  className="text-primary"
+                />
+                <div>
+                  <p className="text-sm text-foreground">Scale</p>
+                  <p className="text-xs text-muted-foreground">
+                    Stretch/shrink content to fit new size
+                  </p>
+                </div>
+              </label>
+
+              <label className="flex items-center gap-2 cursor-pointer">
+                <input
+                  type="radio"
+                  name="resizeMethod"
+                  value="crop"
+                  checked={resizeMethod === 'crop'}
+                  onChange={() => setResizeMethod('crop')}
+                  className="text-primary"
+                />
+                <div>
+                  <p className="text-sm text-foreground">Crop</p>
+                  <p className="text-xs text-muted-foreground">
+                    Crop or center content without scaling
+                  </p>
+                </div>
+              </label>
+
+              <label className="flex items-center gap-2 cursor-pointer">
+                <input
+                  type="radio"
+                  name="resizeMethod"
+                  value="expand"
+                  checked={resizeMethod === 'expand'}
+                  onChange={() => setResizeMethod('expand')}
+                  className="text-primary"
+                />
+                <div>
+                  <p className="text-sm text-foreground">Expand</p>
+                  <p className="text-xs text-muted-foreground">
+                    Expand canvas, center existing content
+                  </p>
+                </div>
+              </label>
+            </div>
+          </div>
+        </div>
+
+        {/* Footer */}
+        <div className="flex justify-end gap-2 border-t border-border p-4">
+          <button
+            onClick={onClose}
+            className="px-4 py-2 text-sm bg-secondary text-secondary-foreground rounded-md hover:bg-secondary/80 transition-colors"
+          >
+            Cancel
+          </button>
+          <button
+            onClick={handleApply}
+            className="px-4 py-2 text-sm bg-primary text-primary-foreground rounded-md hover:bg-primary/90 transition-colors"
+          >
+            Apply
+          </button>
+        </div>
+      </div>
+    </div>
+  );
+}