feat: seeking support and Last.fm scrobbling

Seeking:
- Rust player: seek_requested/seek_target_secs atomics on PlayerStatus
- Decoder loop checks for seek each iteration, calls format.seek() and resets decoder
- New qobuz_backend_seek C FFI + QobuzBackend::seek(quint64)
- Progress slider onProgressReleased now seeks to the dragged position

Last.fm:
- LastFmScrobbler: now-playing + scrobble (50% or 240s threshold, min 30s)
- API signature follows Last.fm spec (sorted params, md5)
- Settings dialog: API key/secret, username/password, Connect button with status
- AppSettings: lastfm/enabled, api_key, api_secret, session_key
- Scrobbler wired to trackChanged, positionChanged, trackFinished in MainWindow

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

src/mainwindow.cpp

@@ -60,6 +60,15 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     setupMenuBar();
     statusBar()->showMessage(tr("Ready"));
 
+    // ---- Scrobbler ----
+    m_scrobbler = new LastFmScrobbler(this);
+    connect(m_backend, &QobuzBackend::trackChanged,
+            m_scrobbler, &LastFmScrobbler::onTrackStarted);
+    connect(m_backend, &QobuzBackend::positionChanged,
+            m_scrobbler, &LastFmScrobbler::onPositionChanged);
+    connect(m_backend, &QobuzBackend::trackFinished,
+            m_scrobbler, &LastFmScrobbler::onTrackFinished);
+
     // ---- Backend signals ----
     connect(m_backend, &QobuzBackend::loginSuccess,   this, &MainWindow::onLoginSuccess);
     connect(m_backend, &QobuzBackend::loginError,     this, &MainWindow::onLoginError);