feat(blocks): YoutubeVideoGalleryBlock (carousel + grid)

New block for `youtube_video_gallery` CMS type. Carousel renders
2 iframes side-by-side on md+ (1 on mobile) with prev/next/dots; grid
shows YT thumbnails opening modal embeds. Description supports
markdown via marked; long URLs break with overflow-wrap:anywhere.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/ContentRows.svelte

@@ -7,6 +7,7 @@
   import ImageGalleryBlock from "./blocks/ImageGalleryBlock.svelte";
   import FilesBlock from "./blocks/FilesBlock.svelte";
   import YoutubeVideoBlock from "./blocks/YoutubeVideoBlock.svelte";
+  import YoutubeVideoGalleryBlock from "./blocks/YoutubeVideoGalleryBlock.svelte";
   import QuoteBlock from "./blocks/QuoteBlock.svelte";
   import QuoteCarouselBlock from "./blocks/QuoteCarouselBlock.svelte";
   import LinkListBlock from "./blocks/LinkListBlock.svelte";
@@ -30,6 +31,7 @@
     ImageGalleryBlockData,
     FilesBlockData,
     YoutubeVideoBlockData,
+    YoutubeVideoGalleryBlockData,
     QuoteBlockData,
     QuoteCarouselBlockData,
     LinkListBlockData,
@@ -118,6 +120,8 @@
                 <FilesBlock block={item as FilesBlockData} />
               {:else if blockType(item) === "youtube_video"}
                 <YoutubeVideoBlock block={item as YoutubeVideoBlockData} />
+              {:else if blockType(item) === "youtube_video_gallery"}
+                <YoutubeVideoGalleryBlock block={item as YoutubeVideoGalleryBlockData} />
               {:else if blockType(item) === "quote"}
                 <QuoteBlock block={item as QuoteBlockData} />
               {:else if blockType(item) === "quote_carousel"}