fix: 恢复 SidebarRail 拖拽鼠标样式，移除右上角主题设置弹窗

- 恢复 SidebarRail 的 cursor-w-resize/cursor-e-resize 样式
- 移除右上角的 ThemePopover 组件

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/layouts/admin.vue

@@ -5,7 +5,6 @@ import { useRouter } from 'vue-router'
 
 import AdminSidebar from '@/components/admin-sidebar/index.vue'
 import LanguageChange from '@/components/language-change.vue'
-import ThemePopover from '@/components/custom-theme/theme-popover.vue'
 import ToggleTheme from '@/components/toggle-theme.vue'
 import api from '@/services/api'
 import { getFileUrl } from '@/utils/config'
@@ -172,7 +171,6 @@ onUnmounted(() => {
         <div class="ml-auto flex items-center gap-2">
           <LanguageChange />
           <ToggleTheme />
-          <ThemePopover />
         </div>
       </header>