fix: add synchronization to User audio fields to prevent data races

User.AudioSource, Boost, Volume, and LocallyMuted were accessed from
both the OnAudioStream audio goroutine and the UI goroutine without
synchronization, a data race under the Go memory model.

Replace direct field access with thread-safe getter/setter methods
protected by a per-user mutex:
- SetAudioSource/GetAudioSource for the OpenAL source pointer
- SetBoost/Boost for the audio boost multiplier
- SetVolume/Volume for the volume level
- SetLocallyMuted/LocallyMuted for the local mute state

Update all call sites across config/, barnard.go, client.go,
ui_tree.go, and stream.go.

client.go

@@ -179,11 +179,11 @@ func (b *Barnard) OnUserChange(e *gumble.UserChangeEvent) {
 			// If the channel is muted, ensure the user is muted
 			if b.MutedChannels[e.User.Channel.ID] {
 				// Only mute if not already muted
-				if !e.User.LocallyMuted {
+				if !e.User.LocallyMuted() {
 					b.UserConfig.ToggleMute(e.User)
 				}
-				if e.User.AudioSource != nil {
-					e.User.AudioSource.SetGain(0)
+				if au := e.User.AudioSource(); au != nil {
+					au.SetGain(0)
 				}
 			}
 		}