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/player/mod.rs

@@ -44,6 +44,9 @@ pub struct PlayerStatus {
     pub current_track: Arc<std::sync::Mutex<Option<TrackDto>>>,
     /// Set to true by the decode thread when a track finishes naturally.
     pub track_finished: Arc<AtomicBool>,
+    /// Set by the player loop when a seek command arrives; cleared by the decode thread.
+    pub seek_requested: Arc<AtomicBool>,
+    pub seek_target_secs: Arc<AtomicU64>,
 }
 
 impl PlayerStatus {
@@ -55,6 +58,8 @@ impl PlayerStatus {
             volume: Arc::new(AtomicU8::new(80)),
             current_track: Arc::new(std::sync::Mutex::new(None)),
             track_finished: Arc::new(AtomicBool::new(false)),
+            seek_requested: Arc::new(AtomicBool::new(false)),
+            seek_target_secs: Arc::new(AtomicU64::new(0)),
         }
     }
 
@@ -121,6 +126,11 @@ impl Player {
         self.status.volume.store(vol, Ordering::Relaxed);
         self.send(PlayerCommand::SetVolume(vol));
     }
+
+    pub fn seek(&self, secs: u64) {
+        self.status.seek_target_secs.store(secs, Ordering::Relaxed);
+        self.status.seek_requested.store(true, Ordering::SeqCst);
+    }
 }
 
 fn player_loop(rx: std::sync::mpsc::Receiver<PlayerCommand>, status: PlayerStatus) {
@@ -177,7 +187,11 @@ fn player_loop(rx: std::sync::mpsc::Receiver<PlayerCommand>, status: PlayerStatu
                     status.position_secs.store(0, Ordering::Relaxed);
                     status.duration_secs.store(0, Ordering::Relaxed);
                 }
-                PlayerCommand::SetVolume(_) | PlayerCommand::Seek(_) => {}
+                PlayerCommand::SetVolume(_) => {}
+                PlayerCommand::Seek(secs) => {
+                    status.seek_target_secs.store(secs, Ordering::Relaxed);
+                    status.seek_requested.store(true, Ordering::SeqCst);
+                }
             },
             Err(std::sync::mpsc::RecvTimeoutError::Timeout) => {}
             Err(std::sync::mpsc::RecvTimeoutError::Disconnected) => break,