style: 优化系列导航栏的样式与折叠箭头，使其与 admonition 保持一致

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

@@ -139,21 +139,6 @@
 							<a href={resolve('/series/[slug]', { slug: data.series.slug })}>{data.series.name}</a
 							></span
 						>
-						<span class="details-icon">
-							<svg
-								aria-hidden="true"
-								focusable="false"
-								role="img"
-								xmlns="http://www.w3.org/2000/svg"
-								viewBox="0 0 256 512"
-								width="12"
-								height="12"
-								><path
-									fill="currentColor"
-									d="M224.3 273l-136 136c-9.4 9.4-24.6 9.4-33.9 0l-22.6-22.6c-9.4-9.4-9.4-24.6 0-33.9l96.4-96.4-96.4-96.4c-9.4-9.4-9.4-24.6 0-33.9L54.3 103c9.4-9.4 24.6-9.4 33.9 0l136 136c9.5 9.4 9.5 24.6.1 34z"
-								></path></svg
-							>
-						</span>
 					</div>
 					<div bind:this={seriesContentEl} class="details-content series-content">
 						<nav>
@@ -237,33 +222,38 @@
 		margin: 0.8rem 0;
 	}
 
-	/* .series-title — 高度对齐 admonition-title */
+	/* .series-title — 高度、字号及箭头与 admonition-title 完全一致 */
 	.series-nav .series-title {
+		position: relative;
 		font-size: 1rem;
 		font-weight: bold;
 		display: flex;
-		justify-content: space-between;
 		align-items: center;
 		line-height: 1.6;
-		padding: 0.25rem 0.75rem;
+		padding: 0.25rem 2rem 0.25rem 0.75rem;
 		background: #e6e6e6;
 		cursor: pointer;
 		user-select: none;
 	}
 
-	/* .details-icon — 对应 DoIt .details .details-icon > svg */
-	.series-nav .details-icon {
-		display: flex;
-		align-items: center;
-		color: #4d4d4d;
+	/* 展开收缩箭头 — 与 admonition 完全一致 */
+	.series-nav .series-title::after {
+		content: '';
+		position: absolute;
+		right: 0.75rem;
+		top: 0.85rem;
+		width: 0.4rem;
+		height: 0.4rem;
+		border-right: 2px solid currentColor;
+		border-bottom: 2px solid currentColor;
+		transform: rotate(-45deg);
+		transition: transform 0.2s ease-in-out;
+		opacity: 0.6;
 	}
 
-	.series-nav .details-icon > :global(svg) {
-		transition: transform 0.2s ease;
-	}
-
-	.series-nav.open .details-icon > :global(svg) {
-		transform: rotate(90deg);
+	.series-nav.open .series-title::after {
+		transform: rotate(45deg);
+		top: 0.7rem;
 	}
 
 	/* .series-content — 展开收缩动画，与 admonition 完全一致 */