fix(frontend): login P0 — 去卡头重复、offline 文案、显密可键盘

去掉卡内盾牌标题；后端不可用改为「服务暂不可用」并禁用登录钮；显密切换可 Tab 聚焦。

frontend/src/pages/login/index.tsx

@@ -1,7 +1,7 @@
 import { memo, useCallback, useEffect, useState } from 'react'
 import { useTranslation } from 'react-i18next'
 import { useLocation, useNavigate } from 'react-router'
-import { Activity, Eye, EyeOff, Shield, ServerOff } from 'lucide-react'
+import { Activity, Eye, EyeOff, ServerOff } from 'lucide-react'
 import { Button } from '@/components/ui/button'
 import { Checkbox } from '@/components/ui/checkbox'
 import { Input } from '@/components/ui/input'
@@ -106,20 +106,12 @@ const Login = memo(function Login() {
             <div>
               <p className="text-sm font-medium text-warning">{t('login.backendDown')}</p>
               <p className="text-xs text-warning mt-1">{t('login.backendDownHint')}</p>
-              <code className="block mt-2 px-3 py-2 bg-warning/20 rounded text-xs font-mono text-warning">
-                ./run.sh
-              </code>
             </div>
           </div>
         )}
 
         {/* Login Card */}
         <div className="bg-card rounded-2xl shadow-lg border border-border p-8">
-          <div className="flex items-center gap-2 mb-6">
-            <Shield className="w-5 h-5 text-primary" />
-            <h2 className="text-lg font-semibold text-foreground">{t('login.heading')}</h2>
-          </div>
-
           {error && (
             <div className="mb-4 p-3 rounded-lg bg-destructive/10 border border-destructive/30 text-destructive text-sm">
               {error}
@@ -157,7 +149,7 @@ const Login = memo(function Login() {
                   type="button"
                   onClick={togglePassword}
                   className="absolute right-3 top-1/2 -translate-y-1/2 text-muted-foreground hover:text-foreground"
-                  tabIndex={-1}
+                  aria-label={showPassword ? t('login.hidePassword') : t('login.showPassword')}
                 >
                   {showPassword ? (
                     <EyeOff className="w-4 h-4" />
@@ -182,7 +174,7 @@ const Login = memo(function Login() {
             <Button
               type="submit"
               className="w-full h-10 bg-primary hover:bg-primary/90 text-primary-foreground"
-              disabled={loading}
+              disabled={loading || backendStatus === 'offline'}
             >
               {loading ? t('login.submitting') : t('login.submit')}
             </Button>