refactor(chat): decouple chat state from auth and gate quota alerts to guests

- Remove ChatAuthStatusChanged dispatch from auth login flow; chat screen now subscribes directly to auth state machine (loginStatus) to derive isGuest and drive WS lifecycle
- Gate quota-exceeded and warning dialogs to guests only (non-guests have no message limit)
- Dispatch ChatLoadMoreHistory for non-guest initial load vs ChatInit for guests
- Fix AppConstants import path from @/core/constants/app_constants to @/core/app_constants
- Add defensive isGuest checks in quota effects to prevent non-guest quota triggers

src/stores/chat/chat-events.ts

@@ -2,6 +2,10 @@
  * Chat 状态机：事件联合
  *
  * 事件名与原 Dart ChatEvent 对齐。
+ *
+ * 历史：原本有 `ChatAuthStatusChanged`（chat 机器刷新 isGuest）—— 已**删**。
+ * 鉴权状态变化（loginStatus）由 chat-screen 通过 `useAuthState()` 订阅，
+ * chat 机器**不**感知鉴权。
  */
 export type ChatEvent =
   | { type: "ChatInit" }
@@ -19,5 +23,4 @@ export type ChatEvent =
     }
   | { type: "ChatWebSocketError"; errorMessage: string }
   | { type: "ChatWebSocketConnected"; userId: string }
-  | { type: "ChatQuotaExceeded" }
-  | { type: "ChatAuthStatusChanged" };
+  | { type: "ChatQuotaExceeded" };