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/index.css

@@ -136,3 +136,43 @@
     font-size: 2.25rem;
   }
 }
+
+/* 骨架屏 shimmer 动画 */
+.skeleton-shimmer {
+  position: relative;
+  overflow: hidden;
+  background: #e5e7eb;
+}
+
+.skeleton-shimmer::after {
+  content: '';
+  position: absolute;
+  inset: 0;
+  background: linear-gradient(
+    90deg,
+    transparent 0%,
+    rgba(255, 255, 255, 0.4) 50%,
+    transparent 100%
+  );
+  animation: shimmer 1.5s infinite;
+  transform: translateX(-100%);
+}
+
+:root.dark .skeleton-shimmer {
+  background: #262626;
+}
+
+:root.dark .skeleton-shimmer::after {
+  background: linear-gradient(
+    90deg,
+    transparent 0%,
+    rgba(255, 255, 255, 0.06) 50%,
+    transparent 100%
+  );
+}
+
+@keyframes shimmer {
+  100% {
+    transform: translateX(100%);
+  }
+}