feat(web): Follow button + clickable artist in album modals

AlbumModal showed the artist name as static text — no way to follow the artist
or jump to them. Adds a Follow affordance (todo #13):

- AlbumInfo gains optional `artistMbid`. When present, the modal title links the
  artist name to /discover/artist/{mbid} and shows a Follow/Following button that
  reflects live follow state.
- Follow state fetched via a new lightweight GET /api/artists?mbid= check
  (avoids pulling the full watched list + discography); Follow reuses
  POST /api/artists. Non-ok POST surfaces a toast (also chips at todo #14).
- Wired artistMbid in the three discovery contexts where following is valuable:
  Discover, Discover artist preview, and Last.fm. Library/discography callers
  pass none (already-followed artists) so the button stays hidden — graceful.

web 152 tests (the ?mbid= follow-state check asserted), tsc + build clean.

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

web/src/app/lastfm/lastfm-client.tsx

@@ -338,6 +338,7 @@ export function LastfmClient() {
                 artist: albumTarget.match.artistName,
                 year: albumTarget.match.firstReleaseDate?.slice(0, 4) ?? null,
                 type: albumTarget.match.primaryType,
+                artistMbid: albumTarget.match.artistMbid,
               }}
               status={
                 owned ? (