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/list/library.cpp

@@ -3,6 +3,9 @@
 #include <QHeaderView>
 #include <QJsonArray>
 #include <QJsonObject>
+#include <QInputDialog>
+#include <QMenu>
+#include <QMessageBox>
 
 namespace List
 {
@@ -24,16 +27,29 @@ Library::Library(QobuzBackend *backend, QWidget *parent)
 {
     setHeaderHidden(true);
     setRootIsDecorated(true);
-    setContextMenuPolicy(Qt::NoContextMenu);
+    setContextMenuPolicy(Qt::CustomContextMenu);
 
     buildStaticNodes();
 
     connect(m_backend, &QobuzBackend::userPlaylistsLoaded,
             this, &Library::onUserPlaylistsLoaded);
+    connect(m_backend, &QobuzBackend::playlistCreated,
+            this, [this](const QJsonObject &) { refresh(); });
+    connect(m_backend, &QobuzBackend::playlistDeleted,
+            this, [this](const QJsonObject &result) {
+        const qint64 deletedId = static_cast<qint64>(result["playlist_id"].toDouble());
+        if (deletedId == m_openPlaylistId) {
+            m_openPlaylistId = 0;
+            emit openPlaylistDeleted();
+        }
+        refresh();
+    });
     connect(this, &QTreeWidget::itemClicked,
             this, &Library::onItemClicked);
     connect(this, &QTreeWidget::itemDoubleClicked,
             this, &Library::onItemDoubleClicked);
+    connect(this, &QTreeWidget::customContextMenuRequested,
+            this, &Library::onContextMenuRequested);
 }
 
 void Library::buildStaticNodes()
@@ -70,6 +86,7 @@ void Library::onUserPlaylistsLoaded(const QJsonObject &result)
     while (m_playlistsNode->childCount() > 0)
         delete m_playlistsNode->takeChild(0);
 
+    QVector<QPair<qint64, QString>> playlists;
     const QJsonArray items = result["items"].toArray();
     for (const auto &v : items) {
         const QJsonObject pl = v.toObject();
@@ -80,7 +97,55 @@ void Library::onUserPlaylistsLoaded(const QJsonObject &result)
         item->setData(0, TypeRole, NodePlaylist);
         item->setData(0, IdRole,   id);
         item->setData(0, NameRole, name);
+
+        playlists.append({id, name});
     }
+
+    emit userPlaylistsChanged(playlists);
+}
+
+void Library::onContextMenuRequested(const QPoint &pos)
+{
+    QTreeWidgetItem *item = itemAt(pos);
+    if (!item) return;
+
+    const bool isHeader = (item == m_playlistsNode);
+    const bool isPlaylistItem = (!isHeader && item->parent() == m_playlistsNode &&
+                                  item->data(0, TypeRole).toInt() == NodePlaylist);
+
+    if (!isHeader && !isPlaylistItem) return;
+
+    QMenu menu(this);
+
+    auto *newPl = menu.addAction(tr("New Playlist…"));
+    connect(newPl, &QAction::triggered, this, [this] {
+        bool ok = false;
+        const QString name = QInputDialog::getText(
+            this, tr("New Playlist"), tr("Playlist name:"),
+            QLineEdit::Normal, QString(), &ok);
+        if (ok && !name.trimmed().isEmpty())
+            m_backend->createPlaylist(name.trimmed());
+    });
+
+    if (isPlaylistItem) {
+        const qint64  plId   = item->data(0, IdRole).toLongLong();
+        const QString plName = item->data(0, NameRole).toString();
+
+        menu.addSeparator();
+        auto *delPl = menu.addAction(tr("Delete \"%1\"…").arg(plName));
+        connect(delPl, &QAction::triggered, this, [this, plId, plName] {
+            const auto answer = QMessageBox::question(
+                this,
+                tr("Delete Playlist"),
+                tr("Permanently delete \"%1\"? This cannot be undone.").arg(plName),
+                QMessageBox::Yes | QMessageBox::Cancel,
+                QMessageBox::Cancel);
+            if (answer == QMessageBox::Yes)
+                m_backend->deletePlaylist(plId);
+        });
+    }
+
+    menu.exec(viewport()->mapToGlobal(pos));
 }
 
 void Library::onItemClicked(QTreeWidgetItem *item, int)
@@ -95,6 +160,7 @@ void Library::onItemClicked(QTreeWidgetItem *item, int)
     case NodePlaylist: {
         const qint64  id   = item->data(0, IdRole).toLongLong();
         const QString name = item->data(0, NameRole).toString();
+        m_openPlaylistId = id;
         emit playlistRequested(id, name);
         break;
     }