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/js/boot.js

@@ -5,10 +5,15 @@
 
   function readTheme() {
     try {
-      return global.localStorage.getItem(THEME_KEY) === "dark" ? "dark" : "light";
+      const stored = global.localStorage.getItem(THEME_KEY);
+      if (stored === "dark" || stored === "light") return stored;
     } catch (_error) {
       return "light";
     }
+    if (global.matchMedia && global.matchMedia("(prefers-color-scheme: dark)").matches) {
+      return "dark";
+    }
+    return "light";
   }
 
   function applyTheme(theme) {