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/dialog/settings.cpp

@@ -36,6 +36,15 @@ SettingsDialog::SettingsDialog(QWidget *parent) : QDialog(parent)
         }
     }
     playLayout->addRow(tr("Preferred quality:"), m_formatBox);
+
+    m_replayGain = new QCheckBox(tr("Enable ReplayGain (track gain normalisation)"), playGroup);
+    m_replayGain->setChecked(AppSettings::instance().replayGainEnabled());
+    playLayout->addRow(m_replayGain);
+
+    m_gapless = new QCheckBox(tr("Gapless playback (pre-fetch next track)"), playGroup);
+    m_gapless->setChecked(AppSettings::instance().gaplessEnabled());
+    playLayout->addRow(m_gapless);
+
     layout->addWidget(playGroup);
 
     // --- Last.fm group ---
@@ -90,6 +99,8 @@ SettingsDialog::SettingsDialog(QWidget *parent) : QDialog(parent)
 void SettingsDialog::applyChanges()
 {
     AppSettings::instance().setPreferredFormat(m_formatBox->currentData().toInt());
+    AppSettings::instance().setReplayGainEnabled(m_replayGain->isChecked());
+    AppSettings::instance().setGaplessEnabled(m_gapless->isChecked());
     AppSettings::instance().setLastFmEnabled(m_lastFmEnabled->isChecked());
     AppSettings::instance().setLastFmApiKey(m_lastFmApiKey->text().trimmed());
     AppSettings::instance().setLastFmApiSecret(m_lastFmApiSecret->text().trimmed());