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/util/settings.hpp

@@ -31,6 +31,19 @@ public:
     bool rememberLogin() const       { return m_settings.value("auth/remember", true).toBool(); }
     void setRememberLogin(bool r)    { m_settings.setValue("auth/remember", r); }
 
+    // --- Last.fm ---
+    bool    lastFmEnabled()   const  { return m_settings.value("lastfm/enabled", false).toBool(); }
+    void    setLastFmEnabled(bool v) { m_settings.setValue("lastfm/enabled", v); }
+
+    QString lastFmApiKey()    const  { return m_settings.value("lastfm/api_key").toString(); }
+    void    setLastFmApiKey(const QString &v)    { m_settings.setValue("lastfm/api_key", v); }
+
+    QString lastFmApiSecret() const  { return m_settings.value("lastfm/api_secret").toString(); }
+    void    setLastFmApiSecret(const QString &v) { m_settings.setValue("lastfm/api_secret", v); }
+
+    QString lastFmSessionKey() const { return m_settings.value("lastfm/session_key").toString(); }
+    void    setLastFmSessionKey(const QString &v){ m_settings.setValue("lastfm/session_key", v); }
+
 private:
     AppSettings() : m_settings("qobuz-qt", "qobuz-qt") {}
     QSettings m_settings;