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.hpp

@@ -8,6 +8,7 @@
 #include "view/queuepanel.hpp"
 #include "view/sidepanel/view.hpp"
 #include "list/library.hpp"
+#include "scrobbler/lastfm.hpp"
 
 #include <QMainWindow>
 #include <QDockWidget>
@@ -51,6 +52,7 @@ private:
     QueuePanel      *m_queuePanel  = nullptr;
     SidePanel::View *m_sidePanel   = nullptr;
     QDockWidget     *m_libraryDock = nullptr;
+    LastFmScrobbler *m_scrobbler   = nullptr;
 
     void setupMenuBar();
     void tryRestoreSession();