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/decoder.rs

@@ -8,10 +8,11 @@ use std::sync::{
 use symphonia::core::{
     codecs::DecoderOptions,
     errors::Error as SymphoniaError,
-    formats::FormatOptions,
+    formats::{FormatOptions, SeekMode, SeekTo},
     io::{MediaSource, MediaSourceStream},
     meta::MetadataOptions,
     probe::Hint,
+    units::Time,
 };
 
 use crate::player::{output::AudioOutput, PlayerStatus};
@@ -172,6 +173,21 @@ pub fn play_track(
             }
         }
 
+        if status.seek_requested.load(Ordering::SeqCst) {
+            status.seek_requested.store(false, Ordering::SeqCst);
+            let target = status.seek_target_secs.load(Ordering::Relaxed);
+            let seeked = format.seek(
+                SeekMode::Coarse,
+                SeekTo::Time { time: Time::from(target), track_id: None },
+            );
+            if let Ok(s) = seeked {
+                let actual = s.actual_ts / sample_rate as u64;
+                status.position_secs.store(actual, Ordering::Relaxed);
+            }
+            decoder.reset();
+            continue;
+        }
+
         let packet = match format.next_packet() {
             Ok(p) => p,
             Err(SymphoniaError::IoError(e)) if e.kind() == std::io::ErrorKind::UnexpectedEof => {