feat: PKCE auth + CI/CD deploy

- Frontend: PKCE flow (oauth.js, api.js centralized, cookie-based AuthContext)
- Backend: token introspection, cookies httpOnly, refresh endpoint
- Replaced localStorage JWT with httpOnly session cookies
- useSaveSync migrated to cookie auth
- cookie-parser added
- Gitea CI workflow (vps-runner pattern)

Backend/package.json

@@ -8,6 +8,7 @@
   },
   "dependencies": {
     "bcrypt": "^5.1.1",
+    "cookie-parser": "^1.4.7",
     "cors": "^2.8.5",
     "dotenv": "^16.4.1",
     "express": "^4.18.2",