feat: per-session typing, incoming toast, remove switched-to messages

- Typing indicator is now per-session (only shows for active session)
- New IncomingToast component: slides down when another session gets a message
  - Shows session name + preview, tappable to switch, auto-dismisses after 4s
- Removed "Switched to X" and "Renamed to X" system messages from chat
- Added sessionId to WsIncomingTyping type

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