fix: keyboard dismiss on tap outside, line breaks, session switch

- GestureDetector wraps Scaffold to dismiss keyboard on tap (keeps text)
- Return key inserts line breaks (TextInputAction.newline)
- Keyboard dismissed after sending and on session switch
- Audio chain playback rewritten with proper queue
- Play button state resets via onPlaybackStateChanged callback
- Empty text bubbles filtered on load

	TODO.md	diff | view | blame | history
	lib/screens/chat_screen.dart	diff | view | blame | history
	lib/widgets/input_bar.dart	diff | view | blame | history