fix: voice transcript display, audio file persistence, debug logging

- Read 'transcript' field from MQTT voice payloads (server uses this, not 'content')
- Save audio to file for cross-session voice messages (survives toJsonLight strip)
- Keep audioUri in toJsonLight when it's a file path (starts with '/')
- Await _storeForSession before showing toast
- Add debug logging to voice handler and store operations

	lib/models/message.dart	diff | view | blame | history
	lib/screens/chat_screen.dart	diff | view | blame | history