fix: CSS brace balance, upgrade deps (0 vulnerabilities), code-splitting

- Fix: styles.css line 1354 double closing brace (CSS syntax error)
- Upgrade: fastify 5.x, @fastify/cors 11, @fastify/static 9, @fastify/websocket 11
- Upgrade: vitest 4.x, vite 8.x (frontend)
- Result: 0 npm vulnerabilities (was 10 backend + 5 frontend)
- Feature: vite.config.ts manualChunks code-splitting (998kB → 5 chunks, max 451kB)
- All tests pass: Backend 239/239, Frontend 374/374

backend/package.json

@@ -11,12 +11,12 @@
     "test": "vitest run"
   },
   "dependencies": {
-    "@fastify/cors": "^9.0.0",
-    "@fastify/static": "^7.0.0",
-    "@fastify/websocket": "^10.0.0",
+    "@fastify/cors": "^11.2.0",
+    "@fastify/static": "^9.1.3",
+    "@fastify/websocket": "^11.2.0",
     "bcrypt": "^6.0.0",
     "better-sqlite3": "^11.0.0",
-    "fastify": "^4.28.0",
+    "fastify": "^5.9.0",
     "y-leveldb": "^0.2.0",
     "y-protocols": "^1.0.7",
     "yjs": "^13.6.0"
@@ -27,6 +27,6 @@
     "@types/node": "^20.14.0",
     "tsx": "^4.16.0",
     "typescript": "^5.5.0",
-    "vitest": "^2.0.0"
+    "vitest": "^4.1.9"
   }
 }