feat(preview): play videos inline in the lightbox overlay

Switch the preview overlay from a still <img> to a <video> tag when the
focused photo's Type is "video". Uses PhotoPrism's /api/v1/videos/:hash
endpoint with the existing previewToken, falls back to a still thumb as
the poster, and autoplays muted so the controls reveal without
clobbering whatever else is on the page.

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

web/src/lib/stores/session.svelte.ts

@@ -87,3 +87,14 @@ export function thumbUrl(hash: string, size = 'tile_500'): string {
 	if (!session.previewToken) return '';
 	return `/api/v1/t/${hash}/${session.previewToken}/${size}`;
 }
+
+/**
+ * Build a video stream URL. PhotoPrism's endpoint is
+ * /api/v1/videos/:hash/:token/:format — same previewToken as thumbnails.
+ * `format=avc` is the standard h264 transcode; HEVC sources are
+ * transcoded on first request and cached server-side.
+ */
+export function videoUrl(hash: string, format = 'avc'): string {
+	if (!session.previewToken) return '';
+	return `/api/v1/videos/${hash}/${session.previewToken}/${format}`;
+}