feat(web): add cover URL helper + fallback SVG data URL

web/src/lib/media/covers.ts

@@ -0,0 +1,16 @@
+// Fallback cover — an inline SVG data URL showing a music-note glyph on the
+// surface color. Used by AlbumCard's onerror handler when the real cover
+// fails (e.g. scanner didn't pick up an embedded image and there's no
+// sidecar). Data URL avoids an extra HTTP round-trip.
+export const FALLBACK_COVER =
+  'data:image/svg+xml;utf8,' +
+  encodeURIComponent(
+    `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+       <rect width="100" height="100" fill="#1f1f23"/>
+       <path d="M40 30v35a7 7 0 1 1-7-7 7 7 0 0 1 3 .7V35h22v8h-18z" fill="#6b6b70"/>
+     </svg>`
+  );
+
+export function coverUrl(albumId: string): string {
+  return `/api/albums/${albumId}/cover`;
+}