Activity sync improvements, bug fixes, auth token reuse, and tests

[jtpatters]

1. Improved Activity Synchronization:

   • The ActivitiesIterator now explicitly fetches activities in ascending order (sort_order='asc') and get_activities_for_date has been updated to handle that order, ensuring a mismatch and failure to write activities does not happen.
   • The sync_status for ACTIVITIES is now correctly updated to completed in _sync_activities_for_date, preventing the re-syncing of already processed activities.

2. Bug Fixes:

   • Timezone-Aware Timestamps: The timestamp_to_datetime utility now returns timezone-aware datetime objects (UTC), ensuring consistency across the library.
   • Data Extraction: The _extract_steps_data method now correctly extracts steps from the last day in daily_steps, and extract_timeseries_data includes a None check for heart_rate.

3. Enhanced Authentication Flow:

   • The SyncManager and garmy-sync CLI now attempt to initialize with saved authentication tokens before prompting for credentials, improving the user experience for repeated syncs (inspired by other outstanding PRs).

4. Testing:

   • A new test file, tests/test_localdb_db.py, has been added with a test suite for the HealthDB class.
   • A test case has been added to verify that the ACTIVITIES sync status is correctly updated.
   • Existing tests have been updated to reflect the changes in the codebase.

5. Build System:

   • The build system has been updated from setuptools to hatchling in pyproject.toml.