feat: multi-image upload and catch_up message delivery

- Multi-image picker with thumbnail strip and caption modal
- Sequence-based catch_up protocol for reliable message delivery
- AppState foreground resume triggers catch_up for missed messages
- Dedup by seq number, notifications suppressed during replay

	app/chat.tsx	diff | view | blame | history
	components/chat/ImageCaptionModal.tsx	diff | view | blame | history
	contexts/ChatContext.tsx	diff | view | blame | history
	types/index.ts	diff | view | blame | history