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

.oxfmtrc.json

@@ -0,0 +1,4 @@
+{
+  "singleQuote": true,
+  "trailingComma": "all"
+}