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-lock.json

@@ -8,6 +8,7 @@
       "name": "content-mentor-frontend",
       "version": "0.1.0",
       "dependencies": {
+        "blurhash": "^2.0.5",
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
         "react-router-dom": "^6.26.0"
@@ -1223,6 +1224,12 @@
         "node": ">=6.0.0"
       }
     },
+    "node_modules/blurhash": {
+      "version": "2.0.5",
+      "resolved": "https://registry.npmjs.org/blurhash/-/blurhash-2.0.5.tgz",
+      "integrity": "sha512-cRygWd7kGBQO3VEhPiTgq4Wc43ctsM+o46urrmPOiuAe+07fzlSB9OJVdpgDL0jPqXUVQ9ht7aq7kxOeJHRK+w==",
+      "license": "MIT"
+    },
     "node_modules/browserslist": {
       "version": "4.28.2",
       "resolved": "https://registry.npmjs.org/browserslist/-/browserslist-4.28.2.tgz",