This commit resolves the persistent `garminconnect` login failure caused by
changes in Garmin's SSO process. The authentication mechanism has been
refactored to primarily use the `garth` library for initial login and
Multi-Factor Authentication (MFA) handling, enhancing robustness and
adhering to the feature plan.
Key changes include:
- Refactored `_perform_login` in `backend/src/services/garmin_auth_service.py`
to directly utilize `garth.Client().login()`, replacing the problematic
`garminconnect.login()`.
- Updated `initial_login` to gracefully handle `garth`'s MFA exceptions,
returning appropriate responses to guide the authentication flow.
- Added a new `complete_mfa_login` method to `backend/src/services/garmin_auth_service.py`
for submitting MFA codes and finalizing the login process.
- Ensured `garminconnect` implicitly leverages the established `garth` session,
eliminating redundant login attempts.
- Addressed static analysis issues by updating `typing` imports and
suppressing `mypy` errors for `garth.Client` attributes where appropriate.
