feat: PDF and document file viewing support

- Add mimeType field to Message model, persisted through JSON
- Pass mimeType from MQTT handler through to message bubbles
- Non-image files (PDF, CSV, Office docs) render as file cards
  with type icon, size info, and tap-to-share via iOS share sheet
- Fix "image decode error" when receiving PDFs via pailot_send_file
- Reconnect backoff: only reset attempt counter after 10s stability

	lib/models/message.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/message_bubble.dart	diff | view | blame | history