feat: show real on-disk tracklist in the Library album modal (#9)

The Library modal showed MusicBrainz's canonical tracklist, hiding incomplete
downloads / edition or track-order mismatches, and showed nothing for albums
without an rgMbid. Now it shows the REAL files on disk.

- LibraryItem gains trackNames String[] (migration add_library_track_names):
  the album's on-disk "## Title.ext" audio filenames.
- Captured at import (pipeline._import lists the final album folder) and at scan
  (scan._record_owned now also refreshes trackNames on re-scan via ON CONFLICT
  DO UPDATE + xmax=0 to preserve the newly-imported flag). New shared
  library.list_audio_files() helper.
- /api/library exposes trackNames; the AlbumModal prefers on-disk tracks when
  present (parsed "## Title" → position/title, zero network, labeled "On disk ·
  N tracks"), falling back to the MusicBrainz listing otherwise. Items imported
  before this column show the MB fallback until re-scanned.

worker 221 tests / 7-skip, web 153, tsc + build clean.

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

web/src/app/design.css

@@ -200,6 +200,10 @@ nav.contents .sep { flex: 1; }
 .modal-artist { display: inline-flex; align-items: center; gap: 12px; flex-wrap: wrap; }
 .modal-artist a { color: inherit; text-decoration: none; border-bottom: 1.5px solid var(--rule-2); }
 .modal-artist a:hover { border-bottom-color: var(--accent); color: var(--accent); }
+.tracks-source {
+  font-family: var(--mono); font-size: 0.62rem; letter-spacing: 0.14em; text-transform: uppercase;
+  color: var(--graphite); margin: 0 0 10px;
+}
 
 /* ── Phase 2: page headers, list rows, tools ──────────── */
 .page-head { margin: 6px 0 26px; }