feat: qbz-1 streaming, gapless prefetch, accurate scrobbling, Range-seek

Port proven playback architecture from qbqt fork:
- Bounded VecDeque buffer with condvar backpressure (4MB cap)
- decrypt_and_extract_frames for clean FLAC frame extraction from ISOBMFF
- Cancel+restart seeking with sub-segment sample skipping
- start_prefetch / QueueNext for gapless transitions with pre-started downloads
- track_transitioned signaling for scrobbler during gapless playback
- Range-request HTTP seeking for non-segmented (MP3) tracks
- OnceLock HTTP client singleton with cancel-aware chunked downloads
- Accumulated listening time scrobbling (prevents false scrobbles from seeking)
- Array-format Last.fm scrobble params (artist[0], track[0], etc.)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/backend/qobuzbackend.cpp

@@ -209,21 +209,6 @@ quint64 QobuzBackend::duration() const { return qobuz_backend_get_duration(m_bac
 int     QobuzBackend::volume()   const { return qobuz_backend_get_volume(m_backend); }
 int     QobuzBackend::state()    const { return qobuz_backend_get_state(m_backend); }
 
-quint32 QobuzBackend::vizRead(float *buf, quint32 maxSamples)
-{
-    return qobuz_backend_viz_read(m_backend, buf, maxSamples);
-}
-
-quint32 QobuzBackend::vizSampleRate() const
-{
-    return qobuz_backend_viz_sample_rate(m_backend);
-}
-
-quint32 QobuzBackend::vizChannels() const
-{
-    return qobuz_backend_viz_channels(m_backend);
-}
-
 // ---- private slots ----
 
 void QobuzBackend::onPositionTick()
@@ -232,6 +217,9 @@ void QobuzBackend::onPositionTick()
 
     if (qobuz_backend_take_track_finished(m_backend))
         emit trackFinished();
+
+    if (qobuz_backend_take_track_transitioned(m_backend))
+        emit trackTransitioned();
 }
 
 void QobuzBackend::onEvent(int eventType, const QString &json)