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/app/splash/components/splash-screen.tsx

@@ -5,7 +5,6 @@ import { useRouter } from "next/navigation";
 
 import { MobileShell } from "@/app/_components/core/mobile-shell";
 import { useAuthDispatch, useAuthState } from "@/stores/auth/auth-context";
-import { useChatDispatch } from "@/stores/chat/chat-context";
 import { useUserDispatch } from "@/stores/user/user-context";
 import { AuthStorage } from "@/data/storage/auth/auth_storage";
 import { ROUTES } from "@/router/routes";
@@ -20,7 +19,6 @@ export function SplashScreen() {
   const router = useRouter();
   const state = useAuthState();
   const authDispatch = useAuthDispatch();
-  const chatDispatch = useChatDispatch();
   const userDispatch = useUserDispatch();
 
   // ─────────────────────────────────────────────────────────────
@@ -66,7 +64,6 @@ export function SplashScreen() {
         "[splash] useEffect ② → router.replace(/chat) [via pendingRedirect flag]",
       );
       authDispatch({ type: "AuthClearPendingRedirect" });
-      chatDispatch({ type: "ChatAuthStatusChanged" });
       userDispatch({ type: "UserInit" });
       router.replace(ROUTES.chat);
     }
@@ -74,7 +71,6 @@ export function SplashScreen() {
     state.loginStatus,
     state.pendingRedirect,
     authDispatch,
-    chatDispatch,
     userDispatch,
     router,
   ]);