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/shared/tokens.css

@@ -146,6 +146,7 @@
   --shadow-float: var(--elevation-float);
   --duration-fast: 150ms;
   --duration-normal: 220ms;
+  --login-brand-gradient: linear-gradient(165deg, #0c1e4a, #16307c, #2153cc);
 
   --font-size-aux: 11.5px;
   --font-size-caption: 12.5px;
@@ -509,6 +510,7 @@
   --warning-line: #6d5a38;
   --warning-line-strong: #66502d;
   --control-shadow: 0 1px 3px rgba(0, 0, 0, .3);
+  --login-brand-gradient: linear-gradient(165deg, #080c18, #0e1730, #14224a);
   --dialog-backdrop: var(--backdrop);
   --ladder-level-1: #2d2426;
   --ladder-level-2: #2b2822;