feat(preview): inline video player via vidstack + hover-warm prefetch

Replace PreviewOverlay's bare <video controls> with a vidstack-driven
player wrapping the same source URL. Vidstack's default video layout
provides a polished chrome (gradient bottom bar, large play overlay,
hover-revealed scrubber) and registers <media-player>/<media-provider>
custom elements that vendor the browser quirks.

To keep first-frame latency low, PhotoTile starts a hover-warm fetch
of the playback URL after a short (120 ms) delay — a single Range
request of the first 512 KB pages the backend's pre-transcoded MP4
cache file into the OS page cache and lands in the browser's HTTP
cache, so when the player mounts and issues its own bytes=0- request
the response is satisfied from disk. Each hash is warmed at most once
per session; AbortController cancels hovers that don't commit.

The vidstack modules are dynamically imported on mount so they never
run during SvelteKit's static prerender — they side-effect
customElements.define() calls which would crash under SSR.

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

@@ -0,0 +1,81 @@
+<!--
+	Vidstack-based video player. Wraps the native `<video>` element with a
+	polished, PhotoPrism-style chrome (gradient bottom bar, large play
+	overlay, hover-revealed scrubber). Element registration is dynamically
+	imported so it never runs during SvelteKit's static prerender.
+
+	Imports are side-effecting:
+	  - `vidstack/player` registers <media-player> / <media-provider>
+	  - `vidstack/player/layouts/default` registers <media-video-layout>
+	  - the two CSS imports ship the dark theme and video-layout styles
+-->
+<script lang="ts">
+	import { onMount } from 'svelte';
+	import 'vidstack/player/styles/default/theme.css';
+	import 'vidstack/player/styles/default/layouts/video.css';
+
+	interface Props {
+		src: string;
+		poster?: string;
+		title?: string;
+	}
+	let { src, poster, title }: Props = $props();
+
+	let ready = $state(false);
+
+	onMount(async () => {
+		// Bundle these into the client chunk only — the modules side-effect
+		// `customElements.define()` calls, which would crash under SSR.
+		await import('vidstack/player');
+		await import('vidstack/player/ui');
+		await import('vidstack/player/layouts/default');
+		ready = true;
+	});
+</script>
+
+{#if ready}
+	<!-- svelte-ignore element_invalid_self_closing_tag -->
+	<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-video-layout></media-video-layout>
+	</media-player>
+{:else}
+	<!-- Fallback while the Vidstack module is loading. Keeps the poster
+		 visible so the transition into the full chrome isn't jarring. -->
+	<!-- svelte-ignore a11y_media_has_caption -->
+	<video
+		{src}
+		{poster}
+		controls
+		autoplay
+		muted
+		playsinline
+		preload="auto"
+		class="max-h-full max-w-full rounded-md object-contain shadow-2xl"
+	></video>
+{/if}
+
+<style>
+	:global(.vds-player) {
+		width: 100%;
+		height: 100%;
+		max-height: 100%;
+		aspect-ratio: auto;
+		--media-brand: #3b82f6;
+		--media-focus-ring-color: #3b82f6;
+	}
+</style>