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/api/subsonicTypes.ts

@@ -22,6 +22,13 @@ export interface SubsonicAlbum {
   releaseTypes?: string[];
   /** OpenSubsonic: album-level credits (Navidrome may attach on album and/or child songs). */
   albumArtists?: SubsonicOpenArtistRef[];
+  /** OpenSubsonic: per-disc subtitles (e.g. "Sessions" on CD 3 of a deluxe edition). */
+  discTitles?: SubsonicDiscTitle[];
+}
+
+export interface SubsonicDiscTitle {
+  disc: number;
+  title: string;
 }
 
 /** OpenSubsonic `artists` / `albumArtists` entries on a child song (may include `userRating`). */