fix(qt): album carousel cards — artist fallback and similar-album genre

Two card-data gaps on the album page's bottom rails:

- "Listening suggestions" (/album/suggest) rendered no artist line:
  the Qt mapping read only the flat `artist` field, while Slint's
  album_map::album_artist falls back to the artists[] array (main-artist
  role, else first entry) — the shape /album/suggest actually returns for
  some albums. The fallback is ported as card_artist.
- "Similar albums" (Last.fm row) showed no genre in the hover overlay:
  AlbumReco carried no genre at all. build_album_reco has the resolving
  Qobuz Album in hand, so the field now exists (serde-defaulted, so
  cached reco JSON keeps working) and is mapped through. Slint's
  external_reco::album_card can map it too after the next merge — its
  checkout is a different branch and is untouched here.

km

crates/qbz-external-reco/src/types.rs

@@ -76,6 +76,11 @@ pub struct AlbumReco {
     pub artwork_url: String,
     #[serde(default)]
     pub subtitle: String,
+    /// Genre name from the resolving Qobuz album — feeds the card hover
+    /// overlay. Defaulted for backward compatibility with cached JSON
+    /// written before the field existed.
+    #[serde(default)]
+    pub genre: String,
     #[serde(default = "default_source")]
     pub source: RecoSource,
 }

crates/qbz-external-reco/src/validate.rs

@@ -284,6 +284,11 @@ pub fn build_album_reco(album: &Album, subtitle: String, source: RecoSource) ->
         quality_label,
         artwork_url: album.image.best().cloned().unwrap_or_default(),
         subtitle,
+        genre: album
+            .genre
+            .as_ref()
+            .map(|g| g.name.clone())
+            .unwrap_or_default(),
         source,
     }
 }