feat(carousel): 2 cols tablet, 3 cols desktop; equal-height cards

- PostOverviewBlock carousel: md:2 cols, lg:3 cols via snap-scroll
- PostCard: add class prop for external styling
- Carousel cards: h-full for equal height

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/components/PostCard.svelte

@@ -22,12 +22,14 @@
     href,
     tagBase = "/posts/tag",
     commentCount = null,
+    class: className = "",
   }: {
     post: PostWithImage;
     href: string;
     tagBase?: string;
     /** Approved comment count. `null` = unknown/loading, hide badge. `0` = also hide. */
     commentCount?: number | null;
+    class?: string;
   } = $props();
 
   const rawImg = $derived(post._rawImageUrl);
@@ -52,7 +54,7 @@
 <Card
   {href}
   layout="post"
-  class={isPastEvent ? "opacity-70 grayscale" : ""}
+  class="{isPastEvent ? 'opacity-70 grayscale' : ''} {className}"
 >
   <div class="relative aspect-3/2 w-full overflow-hidden bg-stein-100">
     {#if rawImg}