Phase 2: Canvas Editor Core

Implements the core canvas editor with react-konva:

- Added dependencies: react-konva, konva, use-image
- DesignCanvas component: 300×300px Stage with T-shirt SVG overlay
- TShirtSVG component: Visual t-shirt outline with print zone indicator
- ImageElement: Draggable/resizable image with Transformer handles
- TextElement: Draggable/resizable text with Transformer handles
- useDesignEditor hook: Element CRUD, selection, reordering
- Keyboard shortcut: Delete/Backspace removes selected element
- Test image added on mount for Phase 2 verification

Canvas info bar shows: "Design Area: 15" × 15" • Export: 4500 × 4500px @ 300 DPI"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/package.json

@@ -11,7 +11,10 @@
   },
   "dependencies": {
     "react": "^19.2.5",
-    "react-dom": "^19.2.5"
+    "react-dom": "^19.2.5",
+    "react-konva": "^18.2.10",
+    "konva": "^9.3.18",
+    "use-image": "^1.1.1"
   },
   "devDependencies": {
     "@eslint/js": "^9.39.4",