feat(timeline): muted hover-to-play preview on video tiles

PhotoPrism plays a silent preview of the actual video when you hover
its grid tile; this mirrors that. After a 250ms debounce the tile
mounts a muted, looping <video> over the thumbnail and cross-fades it
in on first decoded frame, so cursor-skimming doesn't fire N requests
and the tile never blanks mid-fetch. The byte-prefetch helper added
in af96922 is now redundant — the hover <video> warms the same caches
on its own.

Also tells Vidstack the playback URL is video/mp4 via a nested
<source>: our /api/v1/videos/.../avc URL has no extension, so
Vidstack's suffix sniff was failing, falling back to a HEAD probe,
and picking the wrong loader (which surfaced as
NS_ERROR_DOM_MEDIA_METADATA_ERR in Firefox).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/preview/VideoPlayer.svelte

@@ -34,23 +34,29 @@
 </script>
 
 {#if ready}
-	<!-- svelte-ignore element_invalid_self_closing_tag -->
+	<!--
+		Note: `src` is *not* set as an attribute on <media-player>. Our backend
+		URL ends in `/avc` (not `.mp4`), so Vidstack's URL-suffix sniff fails
+		and it falls back to a HEAD probe — which can pick the wrong loader
+		and produce decode errors. Declaring the source via a nested
+		<source type="video/mp4"> tells Vidstack to use the native MP4 loader
+		directly, no probe needed.
+	-->
 	<media-player
 		class="vds-player max-h-full max-w-full rounded-md shadow-2xl"
 		title={title ?? ''}
-		{src}
 		{poster}
 		autoplay
 		muted
 		playsinline
 		load="eager"
 		posterLoad="eager"
-		crossorigin=""
-		preferNativeHLS="false"
 		streamType="on-demand"
 		viewType="video"
 	>
-		<media-provider></media-provider>
+		<media-provider>
+			<source {src} type="video/mp4" />
+		</media-provider>
 		<media-video-layout></media-video-layout>
 	</media-player>
 {:else}