fix: add kuromoji to backend container npm install

The backend uses NODE_PATH=/deps/node_modules with its own npm install
step — not the root package.json. Added kuromoji there and reverted
the incorrect root package.json change.

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

package.json

@@ -7,9 +7,6 @@
     "preview": "vite preview",
     "icons": "node scripts/gen-icons.mjs"
   },
-  "dependencies": {
-    "kuromoji": "^0.1.2"
-  },
   "devDependencies": {
     "sharp": "^0.34.5",
     "vite": "^5.4.0"