fix: downgrade ESLint to v8.x for .eslintrc.json compatibility

ESLint v10.x requires the new flat config format (eslint.config.js),
but the project uses the old .eslintrc.json format. Downgrading to
ESLint v8.x which supports the legacy configuration format.

package.json

@@ -72,7 +72,7 @@
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.1",
     "argon2": "^0.44.0",
-    "eslint": "^10.1.0",
+    "eslint": "^8.57.0",
     "eslint-config-next": "^16.2.1",
     "jsdom": "^29.0.1",
     "tailwindcss": "^4",