style: 优化文章系列导航的样式与展开收缩动画效果

src/routes/posts/[slug]/+page.svelte

@@ -193,14 +193,15 @@
 		margin: 0.8rem 0;
 	}
 
-	/* .series-title — 对应 DoIt .series-title */
+	/* .series-title — 高度对齐 admonition-title */
 	.series-nav .series-title {
 		font-size: 1rem;
 		font-weight: bold;
 		display: flex;
 		justify-content: space-between;
-		line-height: 2em;
-		padding: 0 0.75rem;
+		align-items: center;
+		line-height: 1.6;
+		padding: 0.25rem 0.75rem;
 		background: #e6e6e6;
 		cursor: pointer;
 		list-style: none;
@@ -225,10 +226,23 @@
 		transform: rotate(90deg);
 	}
 
-	/* .series-content — 对应 DoIt .series-content */
+	/* .series-content — 展开收缩动画 */
 	.series-nav .series-content {
 		font-size: 0.875rem;
 		background-color: #f5f5f5;
+		display: grid;
+		grid-template-rows: 0fr;
+		transition: grid-template-rows 0.3s ease-out;
+		overflow: hidden;
+	}
+
+	.series-nav[open] .series-content {
+		grid-template-rows: 1fr;
+	}
+
+	.series-nav .series-content > nav {
+		min-height: 0;
+		overflow: hidden;
 	}
 
 	.series-nav .series-content > nav > ul {