phsl / new-api

* Removed `prompt` prop from `Loading` and switched to built-in Spin indicator with default size `small`
* Dropped overlay background to make the spinner more reusable
* Replaced custom text span; callers can now supply tip via their own UI if needed
* Cleaned up `OAuth2Callback`:
  - Eliminated unused state/variables
  - Added MAX_RETRIES with incremental back-off
  - Centralized error handling via try/catch
  - Streamlined navigation logic on success/failure
  - Updated imports to match new Loading signature

BREAKING CHANGE: `Loading` no longer accepts a `prompt` prop. Update all invocations accordingly.