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/app/chat/components/chat-screen.tsx

@@ -8,7 +8,7 @@
  *   - **订阅 auth 状态机**（loginStatus）——
  *     - 派生 isGuest / 驱动 WS 生命周期 / 驱动初次加载（按 loginStatus 选 ChatInit vs ChatLoadMoreHistory）
  *     - 驱动历史加载
- *   - 屏幕生命周期事件（init / visible / invisible）
+ *   - 屏幕初始化（mount 时按 loginStatus 派初次加载）
  *   - 配额告警监听（**仅**游客 —— "非游客无消息限制"业务事实）
  *   - WebSocket 生命周期管理（基于 auth 状态机的 loginStatus）
  *   - 渲染骨架（背景 + 顶部栏 + 消息区 + 输入栏）
@@ -103,14 +103,10 @@ export function ChatScreen() {
   const [quotaExhausted, setQuotaExhausted] = useState(false);
 
   // ─────────────────────────────────────────────────────────────
-  // 屏幕生命周期 —— mount 时按 loginStatus 派初次加载
+  // mount 时按 loginStatus 派初次加载
   // ─────────────────────────────────────────────────────────────
   useEffect(() => {
-    chatDispatch({ type: "ChatScreenVisible" });
     dispatchInitialLoad(authState.loginStatus, chatDispatch);
-    return () => {
-      chatDispatch({ type: "ChatScreenInvisible" });
-    };
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [chatDispatch]);
   // 注：mount useEffect 故意**不**依赖 loginStatus —— 首次 mount 时机是固定的