fix(youtube-gallery): remove crossfade flash on carousel navigation

The carousel iframe row was wrapped in {#key currentIndex} with
transition:fade, so switching slides (or navigating to the page) faded
the old and new row in overlap — briefly showing two rows / a blink.
Drop the fade from both iframe containers, matching the modal variant
which uses {#key} without a transition.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/blocks/YoutubeVideoGalleryBlock.svelte

@@ -280,7 +280,7 @@
         <div class="grid grid-cols-1 gap-4 md:grid-cols-2">
           {#key currentIndex}
             {@const current = items[currentIndex]}
-            <div class="aspect-video w-full overflow-hidden rounded-xs bg-stein-100" transition:fade={{ duration: 150 }}>
+            <div class="aspect-video w-full overflow-hidden rounded-xs bg-stein-100">
               <iframe
                 title={current.video.title ?? t(T.youtube_title_fallback)}
                 src={current.embedUrl}
@@ -291,7 +291,7 @@
               ></iframe>
             </div>
             {#if second}
-              <div class="hidden md:block aspect-video w-full overflow-hidden rounded-xs bg-stein-100" transition:fade={{ duration: 150 }}>
+              <div class="hidden md:block aspect-video w-full overflow-hidden rounded-xs bg-stein-100">
                 <iframe
                   title={second.video.title ?? t(T.youtube_title_fallback)}
                   src={second.embedUrl}