refactor(ui): 优化骨架屏加载效果，修复用户页左侧黑色背景

- 关键布局组件(AppMenu/TitleBar/SearchBar)改为同步导入，消除加载闪烁
- 新增全局 skeleton-shimmer 流光动画替代 animate-pulse 闪烁效果
- 用户页 loading 骨架屏避免使用 .left scoped 样式导致的深色背景
- 全部 n-skeleton 组件替换为原生 div + shimmer，统一圆角风格
- 菜单容器添加背景色防止加载穿透

src/renderer/views/home/components/HomePlaylistSection.vue

@@ -20,9 +20,9 @@
     <!-- Loading Skeleton -->
     <div v-if="loading" class="grid gap-6" :style="gridStyle">
       <div v-for="i in displayCount" :key="i" class="space-y-3">
-        <div class="aspect-square animate-pulse rounded-2xl bg-neutral-200 dark:bg-neutral-800" />
-        <div class="h-4 w-3/4 animate-pulse rounded bg-neutral-200 dark:bg-neutral-800" />
-        <div class="h-3 w-1/2 animate-pulse rounded bg-neutral-200 dark:bg-neutral-800" />
+        <div class="aspect-square skeleton-shimmer rounded-2xl" />
+        <div class="h-4 w-3/4 skeleton-shimmer rounded-lg" />
+        <div class="h-3 w-1/2 skeleton-shimmer rounded-lg" />
       </div>
     </div>