Implement proper cleanup of BLE scanning operations when users navigate
away from the add-sensor screen to prevent resource waste and potential
issues.
Changes:
- Add useFocusEffect hook to stop BLE scan when screen loses focus
- Remove unused imports (Device, WPDevice, connectDevice, etc.)
- Add comprehensive tests for BLE cleanup behavior
- Add tests for screen unmount/blur scenarios
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Co-Authored-By: Claude <noreply@anthropic.com>