feat: heartbeat fix, copy messages, copy/share images, hide unknown duration

- WebSocket: timestamp-based heartbeat liveness (replaces boolean flag race condition)
- MessageBubble: add Copy option to long-press menu, suppress 0:00 when duration unknown
- ImageViewer: add Copy button alongside Share
- Add expo-clipboard dependency

	bun.lock	diff | view | blame | history
	components/chat/ImageViewer.tsx	diff | view | blame | history
	components/chat/MessageBubble.tsx	diff | view | blame | history
	package.json	diff | view | blame | history
	services/websocket.ts	diff | view | blame | history