施工(HEL-226): 实现登录门户与本机免密切换账号

用设备 Cookie 和授权表记住本机已验证账号，登录页按确认样图做成门户，不再把密码写进浏览器。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

frontend/m/index.html

@@ -15,7 +15,9 @@
       (() => {
         let theme = "light";
         try {
-          theme = localStorage.getItem("xiaobaiTheme") === "dark" ? "dark" : "light";
+          const stored = localStorage.getItem("xiaobaiTheme");
+          if (stored === "dark" || stored === "light") theme = stored;
+          else if (window.matchMedia && window.matchMedia("(prefers-color-scheme: dark)").matches) theme = "dark";
         } catch (_error) {
           theme = "light";
         }