refactor(auth): unify Facebook and Google login into AuthPlatform class

Replace the separate FacebookLogin and GoogleLogin classes with a single
AuthPlatform class that takes a provider name ("facebook" | "google") as a
constructor argument. This consolidates duplicate OAuth sign-in logic behind
one entry point while preserving the existing NextAuth flow.

- Add new src/lib/auth/auth_platform.ts exporting AuthPlatform and
  AuthProvider type
- Update auth-facebook-panel.tsx and splash-button.tsx to use the new API
- Rename initialContext → initialState in the auth machine for consistency
- Update inline docs and comments to reference AuthPlatform

No behavioral change for end users; both providers still route through
NextAuth's signIn() with the same callbacks and cookies.

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

@@ -11,7 +11,7 @@
  *   - 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）
  *   - Skip 用 `<Link>` 保留 SPA 体验
- *   - 社交登录按钮走 `new FacebookLogin().signIn()`（NextAuth 流程）
+ *   - 社交登录按钮走 `new AuthPlatform("facebook").signIn()`（NextAuth 流程）
  */
 import Link from "next/link";
 import { useRouter } from "next/navigation";
@@ -20,7 +20,7 @@ import { useEffect, useRef } from "react";
 import { useAuthState } from "@/stores/auth/auth-context";
 import { useChatDispatch } from "@/stores/chat/chat-context";
 import { useUserDispatch } from "@/stores/user/user-context";
-import { FacebookLogin } from "@/lib/auth/nextauth";
+import { AuthPlatform } from "@/lib/auth/nextauth";
 import { ROUTES } from "@/router/routes";
 import { LoadingIndicator } from "@/app/_components/core/loading-indicator";
 import styles from "./splash-button.module.css";
@@ -53,7 +53,7 @@ export function SplashButton() {
 
   const handleFacebookLogin = async () => {
     try {
-      await new FacebookLogin().signIn();
+      await new AuthPlatform("facebook").signIn();
     } catch (e) {
       console.error("[splash-button] Facebook login failed", e);
     }