feat: blurhash placeholder while image loads

- Add BlurhashCanvas component (decodes hash → canvas pixel data)
- DrawCanvas: expose onImageLoad callback prop
- DrawIt + GenerateIt: show blurhash layer until real image is ready,
  reset imageLoaded state on picture navigation

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

frontend/package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "blurhash": "^2.0.5",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-router-dom": "^6.26.0"