feat(web): AlbumModal follow everywhere + Settings polish (#10)

Follow affordance in ALL album modals (user request): thread artistMbid through
the remaining callers — Library, Wanted, and artist Discography. /api/library
and /api/wanted now expose artistMbid (from the joined MonitoredRelease); the
artist-detail route already exposed mbid. So every album modal now links the
artist name to their page + shows a Follow/Following button.

Settings polish (#10):
- Qobuz tab: drop the email/password inputs (the engine prefers the more
  reliable user-id + auth-token path); a "Clear legacy login" affordance appears
  only when old email/password creds are still stored, and the PUT no longer
  sends them.
- Clear-button alignment fixed: .field label is a flex row and .field-grid
  bottom-aligns inputs, so a set-secret field (with its · set + Clear) lines up
  with a plain sibling field.
- Library tab gains a read-only Staging path panel (/staging, STAGING_DIR, why
  fast local disk, rebuild-to-change).
- Monitor + Discovery tabs gain grounded helptext (quality-class 1/2/3 cutoff,
  poll/retry/upgrade-window; sweep cadence + seeds-per-chunk + ListenBrainz URL).

web 152 tests, tsc + build clean.

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

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

@@ -15,6 +15,7 @@ type Album = {
   rgMbid: string | null;
   year: string | null;
   primaryType: string | null;
+  artistMbid: string | null;
 };
 
 export function LibraryClient() {
@@ -74,7 +75,7 @@ export function LibraryClient() {
         <AlbumModal
           open
           onClose={() => setOpen(null)}
-          album={{ rgMbid: open.rgMbid, album: open.album, artist: open.artist, year: open.year, type: open.primaryType }}
+          album={{ rgMbid: open.rgMbid, album: open.album, artist: open.artist, year: open.year, type: open.primaryType, artistMbid: open.artistMbid }}
           status={<span className="chip done">In library · {open.format}</span>}
         />
       ) : null}