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/components/albumTrackList/AlbumTrackListMobile.tsx

@@ -8,6 +8,7 @@ import { formatLongDuration } from '../../utils/format/formatDuration';
 interface Props {
   discNums: number[];
   discs: Map<number, SubsonicSong[]>;
+  discTitleByNum: Map<number, string>;
   isMultiDisc: boolean;
   currentTrackId: string | null;
   isPlaying: boolean;
@@ -31,6 +32,7 @@ interface Props {
 export function AlbumTrackListMobile({
   discNums,
   discs,
+  discTitleByNum,
   isMultiDisc,
   currentTrackId,
   isPlaying,
@@ -46,6 +48,9 @@ export function AlbumTrackListMobile({
           {isMultiDisc && (
             <div className="disc-header">
               <span className="disc-icon">💿</span> CD {discNum}
+              {discTitleByNum.get(discNum) && (
+                <span className="disc-subtitle">{discTitleByNum.get(discNum)}</span>
+              )}
             </div>
           )}
           {discs.get(discNum)!.map(song => {