refactor(auth): remove AuthGate abstraction in favor of useSession

- Drop AuthGate/AuthGateState re-exports from @/lib/auth/nextauth
- Simplify SplashButton by removing skipHref prop and hardcoding ROUTES.chat
- Update comments to reference useSession() (next-auth/react) directly instead of the AuthGate wrapper, since the auth UI gate no longer needs the extra abstraction layer.

src/app/splash/components/splash-button.tsx

@@ -6,7 +6,7 @@
  *   `Row(children: [Skip 文本, SizedBox 26, Facebook 登录按钮])`
  *
  * 本组件是 splash 鉴权流程的**自治单元**：
- *   - 消费 `useAuthGate`（内部用 useSession 监听 NextAuth session）
+ *   - 消费 `useSession()`（next-auth/react）监听 NextAuth session
  *   - 消费 `useAuthState` / `useAuthDispatch`（邮箱登录流程）
  *   - 消费 `useChatDispatch` / `useUserDispatch`（登录成功后初始化）
  *   - 自管路由跳转（已登录 / 登录成功 → /chat）
@@ -25,12 +25,7 @@ import { ROUTES } from "@/router/routes";
 import { LoadingIndicator } from "@/app/_components/core/loading-indicator";
 import styles from "./splash-button.module.css";
 
-export interface SplashButtonProps {
-  /** Skip 链接的目标路由（默认 /chat） */
-  skipHref?: string;
-}
-
-export function SplashButton({ skipHref = ROUTES.chat }: SplashButtonProps) {
+export function SplashButton() {
   // ===== 鉴权路由跳转已由 src/router/proxy.ts 集中处理 =====
   // 本组件只负责：
   //   1. 触发登录动作
@@ -68,7 +63,7 @@ export function SplashButton({ skipHref = ROUTES.chat }: SplashButtonProps) {
     <div className={styles.wrapper}>
       {/* Skip 文本链接（next/link 渲染为 <a>，保留 SPA 体验） */}
       <Link
-        href={skipHref}
+        href={ROUTES.chat}
         className={styles.skip}
         aria-disabled={isLoading}
         tabIndex={isLoading ? -1 : 0}