feat: OAuth 登录后返回来源页 + 登录页清理

- OAuth 流程透传 redirect 参数，登录后回到触发页面而非固定跳 Dashboard
- 服务端校验 redirect 为相对路径，防止 Open Redirect 攻击
- 隐藏 Apple 登录按钮和邮箱注册入口
- Dark Mode 切换改为下拉菜单样式
- 提取 useClickOutside hook 消除重复代码

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/web/src/components/LanguageToggle.tsx

@@ -1,5 +1,6 @@
-import { useState, useRef, useEffect } from 'react';
+import { useState, useRef, useCallback } from 'react';
 import { useI18n, type Locale } from '../lib/i18n';
+import { useClickOutside } from '../hooks/useClickOutside';
 
 const languages: { locale: Locale; flag: string; label: string }[] = [
   { locale: 'en', flag: '🇺🇸', label: 'English' },
@@ -10,15 +11,7 @@ export default function LanguageToggle() {
   const { locale, setLocale } = useI18n();
   const [open, setOpen] = useState(false);
   const ref = useRef<HTMLDivElement>(null);
-
-  useEffect(() => {
-    if (!open) return;
-    const handler = (e: MouseEvent) => {
-      if (ref.current && !ref.current.contains(e.target as Node)) setOpen(false);
-    };
-    document.addEventListener('mousedown', handler);
-    return () => document.removeEventListener('mousedown', handler);
-  }, [open]);
+  useClickOutside(ref, useCallback(() => setOpen(false), []), open);
 
   return (
     <div ref={ref} className="relative">