feat: session launcher (all-project search + switch-topic) and persistent last-host fast reconnect

	lib/models/project.dart	diff | view | blame | history
	lib/providers/providers.dart	diff | view | blame | history
	lib/screens/chat_screen.dart	diff | view | blame | history
	lib/services/mqtt_service.dart	diff | view | blame | history
	lib/widgets/new_session_sheet.dart	diff | view | blame | history