Fabric.js design canvas implementation and supporting infrastructure

Design Canvas (Primary)
- FabricDesignCanvasV2: Fabric.js-based product customization canvas
  - Lazy-loads Fabric.js to minimize bundle impact
  - Renders product mockup with print area guide (magenta dashed box)
  - Supports front/back view switching with correct product images
  - Color swatch selection for garment customization
  - Text element creation with selection handles and transform controls
  - Fixed React dependency array to prevent infinite renders

Support Components
- DesignCanvasWrapper: Client-side wrapper for SSR compatibility
- DesignCanvasErrorBoundary: Error boundary for canvas failures
- ThemeInitializer: Theme detection and application

Configuration
- Updated .claude/launch.json with dev server settings
- Updated .claude/settings.local.json with local preferences
- Updated next.config.mjs for production build optimization
- Dependency updates in package.json and package-lock.json

Known Issues & TODO
- Image element rendering disabled (Fabric.js compatibility investigation)
- Text editing UI controls needed (font, size, color pickers)
- Delete/undo functionality pending
- Print area alignment needs visual verification
- "Add to Cart" integration with design serialization pending

Testing Artifacts
- check_*.js: Product verification scripts for debugging

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/admin/products/actions.ts

@@ -276,10 +276,10 @@ export async function updateProduct(formData: FormData) {
     referenceWidthCm: number | null;
     referenceHeightCm: number | null;
     weightGrams: number | null;
-    imageUrl?: string;
-    imageUrlBack?: string;
+    imageUrl?: string | null;
+    imageUrlBack?: string | null;
     printArea?: string;
-    printAreaBack?: string;
+    printAreaBack?: string | null;
   } = {
     name,
     category,