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>

rust/src/lib.rs

@@ -402,6 +402,11 @@ pub unsafe extern "C" fn qobuz_backend_set_volume(ptr: *mut Backend, volume: u8)
     (*ptr).0.player.set_volume(volume);
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn qobuz_backend_seek(ptr: *mut Backend, position_secs: u64) {
+    (*ptr).0.player.seek(position_secs);
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn qobuz_backend_get_position(ptr: *const Backend) -> u64 {
     (*ptr).0.player.status.get_position()