refactor: finalize Bluetooth HRM resilience refactor and handle PR feedback

- Verified robust state management and cleanup logic.
- Confirmed named functions for event listeners for correct removal.
- Validated UI consistency and unit system restoration.
- Handled all PR feedback and verified approval.
- All tests passing.

Co-authored-by: arii <342438+arii@users.noreply.github.com>

Author: google-labs-jules[bot]