refactor: improve Bluetooth connection retry and state management

- Increased reconnection attempts to 15 with a hybrid exponential/linear backoff strategy.
- Centralized Bluetooth constants and UUIDs in `constants/bluetooth-config.ts`.
- Implemented a unified `cleanupGattConnection` function for reliable resource cleanup.
- Refined `isConnecting` state management using `try...finally` in connection initiators.
- Added `DISCONNECTING` status and improved manual disconnect handling.
- Restored unit system toggle and fixed height validation display in Imperial mode.
- Enhanced logging for connection aborts and updated unit tests for race conditions.

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

Author: google-labs-jules[bot]