fix: add minimum AudioDataBytes floor for low-bandwidth servers

The auto-bitrate formula 'bandwidth/840 - 42' produces negative values
for servers with bandwidth below ~35 kbps. A negative AudioDataBytes
would cause the Opus encoder to be configured with zero or negative
bitrate, producing invalid output.

Add a floor of 10 bytes per frame (8 kbps), the minimum usable Opus
bitrate for intelligible voice.

gumble/gumbleutil/bitrate.go

@@ -9,10 +9,16 @@ import (
 var autoBitrate = &Listener{
 	Connect: func(e *gumble.ConnectEvent) {
 		if e.MaximumBitrate != nil {
-			const safety = 5
+			const (
+				safety  = 5
+				minBytes = 10 // minimum bytes per frame for usable Opus (8 kbps)
+			)
 			interval := e.Client.Config.AudioInterval
 			dataBytes := (*e.MaximumBitrate / (8 * (int(time.Second/interval) + safety))) - 32 - 10
 
+			if dataBytes < minBytes {
+				dataBytes = minBytes
+			}
 			e.Client.Config.AudioDataBytes = dataBytes
 		}
 	},