feat: most-popular search as default Top Results tab

Add /most-popular/get endpoint support (discovered via Burp):
- Returns mixed artists/albums/tracks ordered by popularity for a query
- New 'Top Results' tab shown first/default in the search panel
- Badge column: A (artist, blue), H (hi-res album, gold), A (album, gray), T (track, green)
- Full context menu support on all item types in the new tab
- Both most-popular and regular search fire on submit in parallel

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

src/backend/qobuzbackend.hpp

@@ -28,6 +28,7 @@ public:
 
     // --- catalog ---
     void search(const QString &query, quint32 offset = 0, quint32 limit = 20);
+    void mostPopularSearch(const QString &query, quint32 limit = 30);
     void getAlbum(const QString &albumId);
     void getArtist(qint64 artistId);
     void getArtistReleases(qint64 artistId, const QString &releaseType, quint32 limit = 50, quint32 offset = 0);
@@ -81,6 +82,7 @@ signals:
 
     // catalog
     void searchResult(const QJsonObject &result);
+    void mostPopularResult(const QJsonObject &result);
     void albumLoaded(const QJsonObject &album);
     void artistLoaded(const QJsonObject &artist);
     void artistReleasesLoaded(const QString &releaseType, const QJsonArray &items, bool hasMore, int offset);