feat: add seamless lazy loading for genre and playlist views

Introduce paged loading with early prefetch for genre albums/playlists and playlist tracks, while preserving full-data behavior for deep shuffle and playlist play-all actions.

src/model/tracklistmodel.cpp

@@ -109,6 +109,92 @@ void TrackListModel::setTracks(const QJsonArray &tracks,
         emit sortApplied();
 }
 
+void TrackListModel::appendTracks(const QJsonArray &tracks,
+                                  bool usePosition,
+                                  bool useSequential)
+{
+    if (tracks.isEmpty())
+        return;
+
+    // Keep append path simple and stable: disc-header mode is handled by reset path.
+    if (m_hasMultipleDiscs && !usePosition && !useSequential) {
+        QJsonArray all = currentTracksJson();
+        for (const QJsonValue &v : tracks)
+            all.append(v);
+        setTracks(all, usePosition, useSequential);
+        return;
+    }
+
+    int seq = 1;
+    if (useSequential || usePosition) {
+        for (const TrackItem &t : m_tracks)
+            if (!t.isDiscHeader)
+                ++seq;
+    }
+
+    QVector<TrackItem> parsed;
+    parsed.reserve(tracks.size());
+    for (const QJsonValue &v : tracks) {
+        const QJsonObject t = v.toObject();
+        TrackItem item;
+        item.id               = static_cast<qint64>(t["id"].toDouble());
+        item.playlistTrackId  = static_cast<qint64>(t["playlist_track_id"].toDouble());
+        item.discNumber       = t["media_number"].toInt(1);
+        item.duration         = static_cast<qint64>(t["duration"].toDouble());
+        item.streamable       = t["streamable"].toBool(true);
+        item.hiRes            = t["hires_streamable"].toBool();
+        item.raw              = t;
+
+        const QString base    = t["title"].toString();
+        const QString version = t["version"].toString().trimmed();
+        item.title = version.isEmpty() ? base
+                   : base + QStringLiteral(" (") + version + QLatin1Char(')');
+
+        if (useSequential) {
+            item.number = seq++;
+        } else if (usePosition) {
+            const int pos = t["position"].toInt();
+            item.number = pos > 0 ? pos : seq;
+            ++seq;
+        } else {
+            item.number = t["track_number"].toInt();
+        }
+
+        const QJsonObject performer = t["performer"].toObject();
+        item.artist = performer["name"].toString();
+        if (item.artist.isEmpty()) {
+            const QJsonValue n = t["album"].toObject()["artist"].toObject()["name"];
+            item.artist = n.isObject() ? n.toObject()["display"].toString() : n.toString();
+        }
+        if (item.artist.isEmpty()) {
+            const QJsonValue n = t["artist"].toObject()["name"];
+            item.artist = n.isObject() ? n.toObject()["display"].toString() : n.toString();
+        }
+
+        const QJsonObject album = t["album"].toObject();
+        item.album   = album["title"].toString();
+        item.albumId = album["id"].toString();
+
+        parsed.append(item);
+    }
+
+    if (parsed.isEmpty())
+        return;
+
+    const int first = m_tracks.size();
+    const int last  = first + parsed.size() - 1;
+    beginInsertRows({}, first, last);
+    m_tracks += parsed;
+    endInsertRows();
+
+    if (!m_hasMultipleDiscs && m_sortColumn >= 0) {
+        emit layoutAboutToBeChanged();
+        sortData(m_sortColumn, m_sortOrder);
+        emit layoutChanged();
+        emit sortApplied();
+    }
+}
+
 void TrackListModel::clear()
 {
     beginResetModel();