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.hpp

@@ -73,11 +73,6 @@ public:
     /// 1 = playing, 2 = paused, 0 = idle
     int     state() const;
 
-    // --- visualizer PCM ---
-    quint32 vizRead(float *buf, quint32 maxSamples);
-    quint32 vizSampleRate() const;
-    quint32 vizChannels() const;
-
 signals:
     // auth
     void loginSuccess(const QString &token, const QJsonObject &user);
@@ -106,6 +101,7 @@ signals:
     void stateChanged(const QString &state);
     void positionChanged(quint64 position, quint64 duration);
     void trackFinished();
+    void trackTransitioned();
 
     // errors
     void error(const QString &message);