chore: CSS优化

- 取消全局Border-box
- 优化布局
- 修复SideBar的异常布局导致divider的height为0不可见问题

src/renderer/src/components/Sidebar.vue

@@ -222,6 +222,7 @@ const togglePlaylists = () => {
   background: linear-gradient(to right, transparent, var(--color-border), transparent);
   margin: 16px 8px;
   opacity: 0.6;
+  flex-shrink: 0;
 }
 
 /* 区域标题 */

src/renderer/src/components/TopBar.vue

@@ -109,6 +109,7 @@ onUnmounted(() => {
 }
 
 .topbar {
+  box-sizing: border-box;
   height: 64px;
   width: 100%;
   display: flex;