feat: refresh views on all playlist mutations

- Add EV_PLAYLIST_TRACK_ADDED (22) — emitted when a track is
  successfully added to a playlist
- If the currently-open playlist is the one modified, re-fetch it
  so the track appears in the list immediately
- After creating a playlist, open it automatically so the user
  lands in the new (empty) playlist view right away
- Sidebar already refreshes on create/delete; this ensures the
  track list view also stays in sync

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

src/mainwindow.cpp

@@ -80,7 +80,13 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     connect(m_backend, &QobuzBackend::playlistLoaded,   this, &MainWindow::onPlaylistLoaded);
     connect(m_backend, &QobuzBackend::playlistCreated,   this, &MainWindow::onPlaylistCreated);
     connect(m_backend, &QobuzBackend::playlistDeleted,   this, [this](const QJsonObject &) {
-        statusBar()->showMessage(tr("Playlist deleted"), 3000);
+        // status bar message is also shown by library's openPlaylistDeleted handler
+    });
+    connect(m_backend, &QobuzBackend::playlistTrackAdded, this, [this](qint64 playlistId) {
+        // Refresh the currently shown playlist if a track was added to it
+        if (m_content->tracksList()->playlistId() == playlistId)
+            m_backend->getPlaylist(playlistId);
+        statusBar()->showMessage(tr("Track added to playlist"), 3000);
     });
     connect(m_backend, &QobuzBackend::trackChanged,   this, &MainWindow::onTrackChanged);
     connect(m_backend, &QobuzBackend::error, this, [this](const QString &msg) {
@@ -337,8 +343,12 @@ void MainWindow::onSearchToggled(bool visible)
 
 void MainWindow::onPlaylistCreated(const QJsonObject &playlist)
 {
-    statusBar()->showMessage(
-        tr("Playlist '%1' created").arg(playlist["name"].toString()), 4000);
+    const QString name = playlist["name"].toString();
+    statusBar()->showMessage(tr("Playlist '%1' created").arg(name), 4000);
+    // Open the new playlist immediately
+    const qint64 id = static_cast<qint64>(playlist["id"].toDouble());
+    if (id > 0)
+        m_backend->getPlaylist(id);
 }
 
 void MainWindow::onUserPlaylistsChanged(const QVector<QPair<qint64, QString>> &playlists)