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/layout/AppLayout.vue

@@ -69,6 +69,10 @@ import { usePlayerStore } from '@/store/modules/player';
 import { useSettingsStore } from '@/store/modules/settings';
 import { isElectron } from '@/utils';
 
+// 关键布局组件同步导入（始终可见，避免加载闪烁）
+import AppMenu from './components/AppMenu.vue';
+import SearchBar from './components/SearchBar.vue';
+import TitleBar from './components/TitleBar.vue';
 // 移动端专用布局
 import MobileLayout from './MobileLayout.vue';
 
@@ -87,11 +91,9 @@ const keepAliveInclude = computed(() => {
     .filter(Boolean);
 });
 
-const AppMenu = defineAsyncComponent(() => import('./components/AppMenu.vue'));
+// 非关键组件保持异步加载
 const PlayBar = defineAsyncComponent(() => import('@/components/player/PlayBar.vue'));
 const MobilePlayBar = defineAsyncComponent(() => import('@/components/player/MobilePlayBar.vue'));
-const SearchBar = defineAsyncComponent(() => import('./components/SearchBar.vue'));
-const TitleBar = defineAsyncComponent(() => import('./components/TitleBar.vue'));
 const PlayingListDrawer = defineAsyncComponent(
   () => import('@/components/player/PlayingListDrawer.vue')
 );
@@ -151,7 +153,7 @@ provide('openPlaylistDrawer', openPlaylistDrawer);
 }
 
 .menu {
-  @apply h-full;
+  @apply h-full bg-light dark:bg-black;
 }
 
 .main {

src/renderer/layout/MobileLayout.vue

@@ -37,9 +37,8 @@ import otherRouter from '@/router/other';
 import { useMenuStore } from '@/store/modules/menu';
 import { usePlayerStore } from '@/store/modules/player';
 
+import AppMenu from './components/AppMenu.vue';
 import MobileHeader from './components/MobileHeader.vue';
-
-const AppMenu = defineAsyncComponent(() => import('./components/AppMenu.vue'));
 const MobilePlayBar = defineAsyncComponent(() => import('@/components/player/MobilePlayBar.vue'));
 const PlayingListDrawer = defineAsyncComponent(
   () => import('@/components/player/PlayingListDrawer.vue')