feat: add message trace log for end-to-end delivery diagnostics

- trace_service.dart: singleton ring buffer (200 entries) for trace events
- trace_screen.dart: log viewer screen with reverse-chron list and upload button
- mqtt_service.dart: _mqttLog adds to TraceService, received/deduped msg traces
- chat_screen.dart: _chatLog adds to TraceService, trace on handleMessage/handleIncomingMessage
- settings_screen.dart: Message Trace Log button navigates to TraceScreen

	lib/screens/chat_screen.dart	diff | view | blame | history
	lib/screens/settings_screen.dart	diff | view | blame | history
	lib/screens/trace_screen.dart	diff | view | blame | history
	lib/services/mqtt_service.dart	diff | view | blame | history
	lib/services/trace_service.dart	diff | view | blame | history