fix: image flicker, screenshot indicator, cross-session message storage

- Cache decoded image bytes to prevent flicker on widget rebuild
- gaplessPlayback prevents blank flash during Image.memory rebuild
- Screenshot button shows snackbar feedback
- Cross-session messages stored to disk immediately (not just toast)
- Messages available when user switches to that session

	lib/screens/chat_screen.dart	diff | view | blame | history
	lib/widgets/message_bubble.dart	diff | view | blame | history