- Clear selected device when audio/video is disabled
- Reset selection if the device disappears on device change
- Prevents DropdownButton “exactly one item with value” crash
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Reset prejoin device selection when a previously chosen audio or video
device becomes unavailable.
* Keep device selection and preview tracks synchronized: auto-select a
device when enabling audio/video if none is selected, and stop/clear
preview tracks when disabling or when inputs are removed.
<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
db929a36d7