feat(library): genre sliders, artist links, and navigation utils

- music landing: diverse per-genre album sliders (online counts /
  offline wide-probe fallback) and home-screen library shortcuts
- add ArtistLinks component and shared navigation/genreDiversity utils
- player/playback-mode refinements across Rust and frontend

src-tauri/src/repository/offline.rs

@@ -864,7 +864,7 @@ impl MediaRepository for OfflineRepository {
 
         let genres = genre_set
             .into_iter()
-            .map(|name| Genre { id: name.clone(), name })
+            .map(|name| Genre { id: name.clone(), name, album_count: None })
             .collect();
 
         Ok(genres)