feat: playlist management, gapless playback, ReplayGain, Qobuz theme

Playlist management:
- Add/remove tracks from playlists via right-click context menu
- Create new playlists (right-click Playlists sidebar header)
- Delete playlists with confirmation dialog (right-click playlist item)
- Playlist view removes track immediately on delete (optimistic)
- Deleting currently-open playlist clears the track view

Gapless playback:
- Single long-running audio thread owns AudioOutput; CPAL stream stays
  open between tracks eliminating device teardown/startup gap
- Decode runs inline on the audio thread; command channel polled via
  try_recv() so Pause/Resume/Seek/Stop/Play all work without spawning
- New Play command arriving mid-decode is handled immediately,
  reusing the same audio output for zero-gap transition
- Position timer reduced from 500 ms to 50 ms for faster track-end detection
- URL/metadata prefetch: when gapless is enabled Qt pre-fetches the next
  track while the current one is still playing

ReplayGain:
- Toggled in Settings → Playback
- replaygain_track_gain (dB) from track audio_info converted to linear
  gain factor and applied per-sample alongside volume

Qobuz dark theme:
- Background #191919, base #141414, accent #FFB232 (yellow-orange)
- Selection highlight, slider fill, scrollbar hover all use #FFB232
- Links use Qobuz blue #46B3EE
- Hi-res H badges updated to #FFB232 (from #FFD700)
- Now-playing row uses #FFB232 (was Spotify green)
- QSS stylesheet for scrollbars, menus, inputs, buttons, groups

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

src/mainwindow.cpp

@@ -77,12 +77,25 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     connect(m_backend, &QobuzBackend::favArtistsLoaded, this, &MainWindow::onFavArtistsLoaded);
     connect(m_backend, &QobuzBackend::albumLoaded,      this, &MainWindow::onAlbumLoaded);
     connect(m_backend, &QobuzBackend::artistLoaded,     this, &MainWindow::onArtistLoaded);
-    connect(m_backend, &QobuzBackend::playlistLoaded, this, &MainWindow::onPlaylistLoaded);
+    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);
+    });
     connect(m_backend, &QobuzBackend::trackChanged,   this, &MainWindow::onTrackChanged);
     connect(m_backend, &QobuzBackend::error, this, [this](const QString &msg) {
         statusBar()->showMessage(tr("Error: %1").arg(msg), 6000);
     });
 
+    // ---- Library signals ----
+    connect(m_library, &List::Library::userPlaylistsChanged,
+            this, &MainWindow::onUserPlaylistsChanged);
+    connect(m_library, &List::Library::openPlaylistDeleted,
+            this, [this] {
+        m_content->showWelcome();
+        statusBar()->showMessage(tr("Playlist deleted"), 3000);
+    });
+
     // ---- Library → backend ----
     connect(m_library, &List::Library::favTracksRequested, this, [this] {
         m_backend->getFavTracks();
@@ -102,9 +115,19 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
         statusBar()->showMessage(tr("Loading playlist: %1…").arg(name));
     });
 
-    // ---- Track list → playback ----
+    // ---- Track list → playback / playlist management ----
     connect(m_content->tracksList(), &List::Tracks::playTrackRequested,
             this, &MainWindow::onPlayTrackRequested);
+    connect(m_content->tracksList(), &List::Tracks::addToPlaylistRequested,
+            this, [this](qint64 trackId, qint64 playlistId) {
+        m_backend->addTrackToPlaylist(playlistId, trackId);
+        statusBar()->showMessage(tr("Adding track to playlist…"), 3000);
+    });
+    connect(m_content->tracksList(), &List::Tracks::removeFromPlaylistRequested,
+            this, [this](qint64 playlistId, qint64 playlistTrackId) {
+        m_backend->deleteTrackFromPlaylist(playlistId, playlistTrackId);
+        statusBar()->showMessage(tr("Removing track from playlist…"), 3000);
+    });
 
     // ---- Search panel ----
     connect(m_sidePanel, &SidePanel::View::albumSelected,
@@ -129,6 +152,9 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     connect(m_toolBar, &MainToolBar::queueToggled,
             this, [this](bool v) { m_queuePanel->setVisible(v); });
 
+    // Apply playback options from saved settings
+    m_backend->setReplayGain(AppSettings::instance().replayGainEnabled());
+
     tryRestoreSession();
 }
 
@@ -233,6 +259,16 @@ void MainWindow::onTrackChanged(const QJsonObject &track)
         : track["performer"].toObject()["name"].toString();
     statusBar()->showMessage(
         artist.isEmpty() ? title : QStringLiteral("▶  %1 — %2").arg(artist, title));
+
+    // Gapless: prefetch next track URL so it starts immediately
+    if (AppSettings::instance().gaplessEnabled() && m_queue->canGoNext()) {
+        const auto upcoming = m_queue->upcomingTracks(1);
+        if (!upcoming.isEmpty()) {
+            const qint64 nextId = static_cast<qint64>(upcoming.first()["id"].toDouble());
+            if (nextId > 0)
+                m_backend->prefetchTrack(nextId, AppSettings::instance().preferredFormat());
+        }
+    }
 }
 
 void MainWindow::onFavTracksLoaded(const QJsonObject &result)
@@ -298,3 +334,15 @@ void MainWindow::onSearchToggled(bool visible)
 {
     m_sidePanel->setVisible(visible);
 }
+
+void MainWindow::onPlaylistCreated(const QJsonObject &playlist)
+{
+    statusBar()->showMessage(
+        tr("Playlist '%1' created").arg(playlist["name"].toString()), 4000);
+}
+
+void MainWindow::onUserPlaylistsChanged(const QVector<QPair<qint64, QString>> &playlists)
+{
+    m_userPlaylists = playlists;
+    m_content->tracksList()->setUserPlaylists(playlists);
+}