fix: reduce audio jitter buffer latency (#673)

* fix: reduce audio jitter buffer latency

- Release listenersMu before dispatching samples to listeners
- Buffer Track.sample channel (size 2) with non-blocking WriteSample
- Buffer pipeline.sample channel (size 4) to unblock GStreamer thread
- Add leaky=downstream max-size-buffers=5 to audio queue to prevent burst accumulation
- Set sync=false on appsink to deliver buffers immediately without clock wait

These changes break the backpressure chain that caused the GStreamer
streaming thread to stall. When the Go scheduler has a hiccup, buffered
frames were delivered in a burst, causing the browser audio jitter buffer
to grow to ~10 seconds over time.

Fixes: #598

* fix: log message when dropping samples due to full track channel

server/internal/capture/manager.go

@@ -135,7 +135,7 @@ func New(desktop types.DesktopManager, config *config.Capture) *CaptureManagerCt
 				"pulsesrc device=%s "+
 					"! audio/x-raw,channels=2 "+
 					"! audioconvert "+
-					"! queue "+
+					"! queue max-size-buffers=5 leaky=downstream "+
 					"! %s "+
 					"! appsink name=appsink", config.AudioDevice, config.AudioCodec.Pipeline,
 			), nil