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/block-types.ts

@@ -168,6 +168,18 @@ export interface YoutubeVideoBlockData {
   layout?: BlockLayout;
 }
 
+/** Galerie aus YouTube-Video-Referenzen (_type: "youtube_video_gallery"). */
+export interface YoutubeVideoGalleryBlockData {
+  _type?: "youtube_video_gallery";
+  _slug?: string;
+  title?: string;
+  subtitle?: string;
+  description?: string;
+  variant?: "carousel" | "grid";
+  videos?: YoutubeVideoBlockData[];
+  layout?: BlockLayout;
+}
+
 /** Zitat (_type: "quote"). */
 export interface QuoteBlockData {
   _type?: "quote";