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/backend/qobuzbackend.cpp

@@ -128,6 +128,11 @@ void QobuzBackend::setVolume(int volume)
     qobuz_backend_set_volume(m_backend, static_cast<quint8>(qBound(0, volume, 100)));
 }
 
+void QobuzBackend::seek(quint64 positionSecs)
+{
+    qobuz_backend_seek(m_backend, positionSecs);
+}
+
 quint64 QobuzBackend::position() const { return qobuz_backend_get_position(m_backend); }
 quint64 QobuzBackend::duration() const { return qobuz_backend_get_duration(m_backend); }
 int     QobuzBackend::volume()   const { return qobuz_backend_get_volume(m_backend); }

src/backend/qobuzbackend.hpp

@@ -48,6 +48,7 @@ public:
     void resume();
     void stop();
     void setVolume(int volume);
+    void seek(quint64 positionSecs);
 
     quint64 position() const;
     quint64 duration() const;