refactor(chat): remove legacy screen visibility events and use absolute state refs

- Remove ChatScreenVisible and ChatScreenInvisible events from chat state machine
- Remove legacy ChatInit event handler
- Migrate state transitions to use absolute references (#chat.guestSession, #chat.userSession, #chat.idle)
- Adjust legal text wording for Cozsweet privacy agreement

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

@@ -96,10 +96,8 @@ export function ChatScreen() {
   // 屏幕生命周期 + 派初次鉴权生命周期事件
   // ─────────────────────────────────────────────────────────────
   useEffect(() => {
-    chatDispatch({ type: "ChatScreenVisible" });
     dispatchAuthLifecycle(authState.loginStatus, chatDispatch);
     return () => {
-      chatDispatch({ type: "ChatScreenInvisible" });
     };
     // 注：mount useEffect 故意**不**依赖 loginStatus —— 首次 mount 时机是固定的
     //     （"用户进 /chat 那一刻"），loginStatus 由 splash/auth-screen 提前 settle。