refactor: 收敛单用户模型并优化可访问性与响应式体验

- 后端移除 is_admin 权限字段与相关逻辑，统一为单用户系统模型
- 修复会话过期清理的时间比较方式（改为 RFC3339 参数比较）
- /api/config 聚合配置增加敏感字段脱敏，避免暴露 TURN/RustDesk 密钥与密码
- 配置更新日志改为摘要，避免打印完整配置内容
- 前端修复可点击卡片语义与键盘可达，补齐图标按钮可访问名称
- 调整弹窗与抽屉的响应式尺寸，优化多端显示与交互

web/src/views/LoginView.vue

@@ -49,7 +49,7 @@ function handleKeydown(e: KeyboardEvent) {
 </script>
 
 <template>
-  <div class="min-h-screen flex items-center justify-center bg-background p-4">
+  <div class="min-h-screen min-h-dvh flex items-center justify-center bg-background p-4">
     <div class="w-full max-w-sm space-y-6">
       <!-- Logo and Title -->
       <div class="text-center space-y-2">
@@ -91,6 +91,7 @@ function handleKeydown(e: KeyboardEvent) {
           <button
             type="button"
             class="absolute right-3 top-1/2 -translate-y-1/2 text-muted-foreground hover:text-foreground"
+            :aria-label="showPassword ? t('extensions.rustdesk.hidePassword') : t('extensions.rustdesk.showPassword')"
             @click="showPassword = !showPassword"
           >
             <Eye v-if="!showPassword" class="w-4 h-4" />