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/components/ui/sidebar/SidebarRail.vue

@@ -21,10 +21,11 @@ const { toggleSidebar } = useSidebar()
     :title="t('common.toggleSidebar')"
     :class="cn(
       'hover:after:bg-sidebar-border absolute inset-y-0 z-20 hidden w-4 -translate-x-1/2 transition-all ease-linear group-data-[side=left]:-right-4 group-data-[side=right]:left-0 after:absolute after:inset-y-0 after:left-1/2 after:w-[2px] sm:flex',
+      'in-data-[side=left]:cursor-w-resize in-data-[side=right]:cursor-e-resize',
+      '[[data-side=left][data-state=collapsed]_&]:cursor-e-resize [[data-side=right][data-state=collapsed]_&]:cursor-w-resize',
       'hover:group-data-[collapsible=offcanvas]:bg-sidebar group-data-[collapsible=offcanvas]:translate-x-0 group-data-[collapsible=offcanvas]:after:left-full',
       '[[data-side=left][data-collapsible=offcanvas]_&]:-right-2',
       '[[data-side=right][data-collapsible=offcanvas]_&]:-left-2',
-      'cursor-pointer',
       props.class,
     )"
     @click="toggleSidebar"