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/dialog/settings.hpp

@@ -3,6 +3,10 @@
 #include <QDialog>
 #include <QComboBox>
 #include <QLabel>
+#include <QLineEdit>
+#include <QCheckBox>
+#include <QPushButton>
+#include <QNetworkAccessManager>
 
 class SettingsDialog : public QDialog
 {
@@ -12,7 +16,19 @@ public:
     explicit SettingsDialog(QWidget *parent = nullptr);
 
 private:
+    // Playback
     QComboBox *m_formatBox = nullptr;
 
+    // Last.fm
+    QCheckBox  *m_lastFmEnabled    = nullptr;
+    QLineEdit  *m_lastFmApiKey     = nullptr;
+    QLineEdit  *m_lastFmApiSecret  = nullptr;
+    QLineEdit  *m_lastFmUsername   = nullptr;
+    QLineEdit  *m_lastFmPassword   = nullptr;
+    QPushButton *m_lastFmConnect   = nullptr;
+    QLabel      *m_lastFmStatus    = nullptr;
+    QNetworkAccessManager *m_nam   = nullptr;
+
     void applyChanges();
+    void onLastFmConnect();
 };