feat: add streaming buffer progress indicator to seekbar

Show a subtle buffer progress bar in the seekbar when streaming
tracks (before they are fully cached). The bar disappears once
the track is fully buffered.

Backend: expose buffer_progress from BufferedMediaSource through
SharedState -> PlaybackEvent -> playerStore.
Frontend: render .seekbar-buffer bar behind the playback fill.

cc

crates/qbz-player/src/player/mod.rs

@@ -602,6 +602,8 @@ pub struct SharedState {
     gapless_ready: Arc<AtomicBool>,
     /// Track ID of the gapless-queued next track (0 = none)
     gapless_next_track_id: Arc<AtomicU64>,
+    /// Streaming buffer progress (0.0-1.0 stored as f32 bits, 0 = not streaming)
+    buffer_progress: Arc<AtomicU32>,
 }
 
 impl Default for SharedState {
@@ -628,6 +630,7 @@ impl SharedState {
             normalization_gain: Arc::new(AtomicU32::new(0)),
             gapless_ready: Arc::new(AtomicBool::new(false)),
             gapless_next_track_id: Arc::new(AtomicU64::new(0)),
+            buffer_progress: Arc::new(AtomicU32::new(0)),
         }
     }
 
@@ -671,6 +674,23 @@ impl SharedState {
         }
     }
 
+    /// Set streaming buffer progress (0.0 to 1.0). Pass 0.0 when not streaming.
+    pub fn set_buffer_progress(&self, progress: f32) {
+        self.buffer_progress
+            .store(progress.to_bits(), Ordering::SeqCst);
+    }
+
+    /// Get streaming buffer progress (0.0 to 1.0). Returns None if not streaming.
+    pub fn get_buffer_progress(&self) -> Option<f32> {
+        let bits = self.buffer_progress.load(Ordering::SeqCst);
+        let progress = f32::from_bits(bits);
+        if progress <= 0.0 || progress >= 1.0 {
+            None
+        } else {
+            Some(progress)
+        }
+    }
+
     pub fn set_current_device(&self, device: Option<String>) {
         if let Ok(mut d) = self.current_device.write() {
             *d = device;
@@ -2314,6 +2334,14 @@ impl Player {
                             if now.duration_since(last_empty_check) >= Duration::from_millis(500) {
                                 last_empty_check = now;
 
+                                // Update streaming buffer progress for UI seekbar
+                                if let Some(streaming_src) = current_streaming_source.as_ref() {
+                                    let progress = streaming_src.progress().unwrap_or(1.0);
+                                    thread_state.set_buffer_progress(progress);
+                                } else {
+                                    thread_state.set_buffer_progress(0.0);
+                                }
+
                                 // Streaming -> cached promotion:
                                 // once streaming download completes, persist full data and clear streaming marker.
                                 // This unlocks normal gapless pre-queue for the current track's tail.