feat(web): Library page — owned-albums grid with cover art

New /library: GET /api/library lists LibraryItems enriched with rgMbid + year
from the matching MonitoredRelease. Client renders a cover-art grid with a live
filter; clicking an album opens the AlbumModal (tracklist). Added Library to the
nav. The album-only overview to see everything downloaded.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/_ui/contents-nav.tsx

@@ -4,6 +4,7 @@ import { usePathname } from "next/navigation";
 
 const LINKS: { href: string; label: string }[] = [
   { href: "/", label: "The Floor" },
+  { href: "/library", label: "Library" },
   { href: "/artists", label: "Artists" },
   { href: "/discover", label: "Discover" },
   { href: "/wanted", label: "Wanted" },