feat(layout): 全量模式右上角也加界面模式切换

复用 appearance-controls 的 LayoutModeSwitcher，Header 操作区在换肤旁
补上界面模式下拉，与简洁模式右上角保持一致。LayoutModeSwitcher 新增
fallback 属性指明 mode 未设置时的兜底界面（Header 传 full，ChatPage 传 compact）。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/strategy-components/Header.tsx

@@ -4,7 +4,7 @@ import { useLocation, useNavigate } from "react-router-dom";
 
 import { Button } from "@/components/ui/button";
 import "@/components/page-sdebar.css";
-import { ThemeSwitcher } from "@/components/workspace/appearance-controls";
+import { LayoutModeSwitcher, ThemeSwitcher } from "@/components/workspace/appearance-controls";
 import { NotificationDialog } from "@/components/workspace/notification-dialog";
 import { getStoredAuth, getAccountDisplayName } from "@/core/auth";
 import { isWujieEnv } from "@/core/embed/wujie";
@@ -126,6 +126,11 @@ const Header = ({ collapsed }: HeaderProps) => {
     <>
       <ThemeSwitcher tone={isDark ? "banner-dark" : "banner-light"} />
 
+      <LayoutModeSwitcher
+        tone={isDark ? "banner-dark" : "banner-light"}
+        fallback="full"
+      />
+
       <Button
         type="button"
         variant="ghost"