Implements RestoreEntity to maintain cover position and state after Home Assistant restarts.
Changes:
- Add RestoreEntity mixin to AirSendCover class
- Implement async_added_to_hass() to restore previous state
- Position restoration for type 4099 covers (with position support)
- Open/closed state restoration for all cover types
- Maintains backward compatibility with 50% default for new covers
Fixes issue where covers would reset to 50% position after every restart.
