refactor: relocate components to app directory structure

src/app/auth/components/google-sign-in-button.tsx

@@ -0,0 +1,50 @@
+"use client";
+/**
+ * Google 登录按钮（GIS 渲染）
+ *
+ * 原始 Dart: lib/ui/auth/widgets/google_sign_in_web.dart
+ *
+ * 通过 `google.accounts.id.renderButton` 把 GIS 官方按钮渲染到指定 div。
+ * 登录成功由 `setOnGoogleSignInSuccess` 设置的回调处理。
+ */
+import { useEffect, useRef } from "react";
+
+import {
+  renderGoogleButton,
+  setOnGoogleSignInSuccess,
+  type GoogleSignInSuccess,
+} from "@/integrations/google-identity";
+import styles from "./google-sign-in-button.module.css";
+
+export interface GoogleSignInButtonProps {
+  onSuccess: (success: GoogleSignInSuccess) => void;
+}
+
+export function GoogleSignInButton({ onSuccess }: GoogleSignInButtonProps) {
+  const ref = useRef<HTMLDivElement | null>(null);
+
+  useEffect(() => {
+    setOnGoogleSignInSuccess(onSuccess);
+    return () => setOnGoogleSignInSuccess(null);
+  }, [onSuccess]);
+
+  useEffect(() => {
+    if (!ref.current) return;
+    let cancelled = false;
+    // GIS 异步加载，轮询直到可用
+    const tryRender = () => {
+      if (cancelled) return;
+      if (window.google?.accounts?.id) {
+        renderGoogleButton(ref.current!, { theme: "outline", size: "large" });
+      } else {
+        setTimeout(tryRender, 100);
+      }
+    };
+    tryRender();
+    return () => {
+      cancelled = true;
+    };
+  }, []);
+
+  return <div ref={ref} className={styles.slot} />;
+}