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/package.json

@@ -34,6 +34,7 @@
 		"mode-watcher": "^1.1.0",
 		"svelte-sonner": "^1.1.1",
 		"tailwind-merge": "^3.6.0",
-		"tailwind-variants": "^3.2.2"
+		"tailwind-variants": "^3.2.2",
+		"vidstack": "^1.12.13"
 	}
 }