feat(album): show OpenSubsonic disc subtitles after the CD heading (#753)

* feat(album): show OpenSubsonic disc subtitles after the CD heading

Multi-disc albums in OpenSubsonic / Navidrome carry a per-disc
subtitle (`discTitles`) — e.g. "Sessions" on CD 3 of a deluxe
edition. AlbumTrackList only rendered "CD N" and dropped the
subtitle, so users couldn't tell two discs apart unless they read
the track names.

* `SubsonicAlbum.discTitles` typed; `getAlbum` forwards it as-is.
* `AlbumTrackList` and `AlbumTrackListMobile` take a discTitleByNum
  map and render the subtitle in the disc separator after "CD N".
* Heading bumped slightly (13 → 15 px, icon 16 → 18 px) so the disc
  separator stays legible next to the new subtitle.

* docs(changelog): note disc subtitles after CD heading (#753)

src/pages/AlbumDetail.tsx

@@ -341,6 +341,7 @@ const handleShuffleAll = () => {
 
       <AlbumTrackList
         songs={displayedSongs}
+        discTitles={album?.album.discTitles}
         sorted={sortKey !== 'natural' || !!filterText.trim()}
         hasVariousArtists={hasVariousArtists}
         currentTrack={currentTrack}