fix(antigravity): refresh expired OAuth token and retry on 401

[tamakichi23]

Summary

• Fix Antigravity usage polling getting stuck in AuthRequired after an access token expires on Windows.
• Use the refresh token from the existing Windows credential to refresh the access token when it is expired or near expiry.
• If loadCodeAssist or quota retrieval returns an authentication error, refresh the token and retry the polling request once.
• Fall back to the existing AuthRequired behavior if refresh fails or the retry is rejected.
• Keep the existing credential unchanged and do not log tokens, OAuth client secrets, or other sensitive data.
• Update the README to accurately describe the OAuth refresh behavior.

Testing

• Antigravity/provider tests: 119 passed.
• Settings tests: 14 passed.
• cargo fmt -- --check: passed.
• cargo clippy: passed.
• cargo build --release --locked: passed.
• git diff --check: passed.
• Manually verified on Windows that Antigravity quota polling recovered from an existing 401 / AuthRequired state without signing in to Antigravity again.
• Full test suite: 434 passed, 3 ignored, 2 failed in unchanged Theme Studio tests:
  • studio_app::tests::diagnostics_page_has_logging_controls_and_menu_version
  • studio_app::tests::version_text_and_trailing_icon_share_one_update_button

Checklist

• I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md.
• The change is focused and contains no credentials or sensitive data.
• I have added or updated relevant documentation and tests, where applicable.
• I have left changelog and version changes to maintainers unless requested.