feat(task3): migrate toolchain to oxfmt + oxlint

- Install oxfmt with .oxfmtrc.json (maps Prettier config)
- Install oxlint with oxlint.config.ts (TypeScript + React rules)
- Replace admin ESLint with oxlint
- Remove Prettier integration from server ESLint (keep ESLint for type checking)
- Delete server .prettierrc and admin eslint.config.js

oxlint.config.ts

@@ -0,0 +1,16 @@
+import type { OxlintConfig } from 'oxlint';
+
+const config: OxlintConfig = {
+  plugins: ['typescript', 'react', 'import'],
+  rules: {
+    'typescript/no-explicit-any': 'off',
+    'typescript/no-non-null-assertion': 'warn',
+  },
+  settings: {
+    react: {
+      version: '19.0.0',
+    },
+  },
+};
+
+export default config;