style: 优化管理后台侧边栏导航布局和间距

src/routes/admin/+layout.svelte

@@ -274,7 +274,7 @@
 	}
 
 	.nav-group {
-		margin-bottom: 30px;
+		margin-bottom: 20px;
 	}
 
 	.nav-group-title {
@@ -284,14 +284,14 @@
 		text-transform: uppercase;
 		letter-spacing: 1px;
 		padding: 0 24px;
-		margin-bottom: 12px;
+		margin-bottom: 8px;
 	}
 
 	.nav-item {
 		display: flex;
 		align-items: center;
 		gap: 12px;
-		padding: 10px 24px;
+		padding: 8px 24px;
 		color: #b0b0b0; /* TOC text color */
 		text-decoration: none;
 		font-size: 0.95rem;