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/hooks/useClickOutside.ts

@@ -0,0 +1,12 @@
+import { useEffect, type RefObject } from 'react';
+
+export function useClickOutside(ref: RefObject<HTMLElement | null>, onClose: () => void, active: boolean) {
+  useEffect(() => {
+    if (!active) return;
+    const handler = (e: MouseEvent) => {
+      if (ref.current && !ref.current.contains(e.target as Node)) onClose();
+    };
+    document.addEventListener('mousedown', handler);
+    return () => document.removeEventListener('mousedown', handler);
+  }, [active, ref, onClose]);
+}