fix: correct top_tracks field name and remove unused extra param

Qobuz API uses snake_case so the field is top_tracks, not topTracks.
Also remove the extra=topTracks query param — top tracks are included
in the default artist/page response without it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/src/api/client.rs

@@ -258,10 +258,7 @@ impl QobuzClient {
     pub async fn get_artist_page(&self, artist_id: i64) -> Result<Value> {
         let resp = self
             .get_request("artist/page")
-            .query(&[
-                ("artist_id", artist_id.to_string()),
-                ("extra",     "topTracks".to_string()),
-            ])
+            .query(&[("artist_id", artist_id.to_string())])
             .send()
             .await?;
         Self::check_response(resp).await