fix: downgrade ESLint to v8.57.1 for LSP compatibility

- Downgrade ESLint from v10.1.0 to v8.57.1
- Create .eslintrc.json config (ESLint v8 format)
- Remove eslint.config.js (flat config format)
- ESLint LSP (vscode-langservers-extracted) doesn't support ESLint v10+

The vscode-eslint-language-server from vscode-langservers-extracted v4.10.0
is not compatible with ESLint v10's Flat Config format. Downgrading to
ESLint v8 ensures the ESLint LSP in Neovim works correctly.

package.json

@@ -74,7 +74,7 @@
     "@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",