refactor(chat): remove unused ChatScreenVisible and ChatScreenInvisible events

The screen-lifecycle events were dead code: the transitions in chat-machine
(idle <-> ready) had no caller behavior tied to them, and the dispatches
in chat-screen.tsx were the only senders. Drop the union members, both
state transitions, and the mount-effect dispatches.

The idle state remains declared as initial but is no longer reachable
at runtime; can be refactored in a follow-up.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/stores/chat/chat-events.ts

@@ -9,8 +9,6 @@
  */
 export type ChatEvent =
   | { type: "ChatInit" }
-  | { type: "ChatScreenVisible" }
-  | { type: "ChatScreenInvisible" }
   | { type: "ChatSendMessage"; content: string }
   | { type: "ChatSendImage"; imageBase64: string }
   | { type: "ChatLoadMoreHistory" }