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.

barnard.go

@@ -115,17 +115,17 @@ func (b *Barnard) TreeItemKeyPress(ui *uiterm.Ui, tree *uiterm.Tree, item uiterm
 			users := makeUsersArray(treeItem.Channel.Users)
 			for _, u := range users {
 				// Explicitly set user mute state to match channel state
-				if channelWillBeMuted && !u.LocallyMuted {
+				if channelWillBeMuted && !u.LocallyMuted() {
 					b.UserConfig.ToggleMute(u)
-				} else if !channelWillBeMuted && u.LocallyMuted {
+				} else if !channelWillBeMuted && u.LocallyMuted() {
 					b.UserConfig.ToggleMute(u)
 				}
 
-				if u.AudioSource != nil {
-					if u.LocallyMuted {
-						u.AudioSource.SetGain(0)
+				if au := u.AudioSource(); au != nil {
+					if u.LocallyMuted() {
+						au.SetGain(0)
 					} else {
-						u.AudioSource.SetGain(u.Volume)
+						au.SetGain(u.Volume())
 					}
 				}
 			}
@@ -159,11 +159,11 @@ func (b *Barnard) TreeItemKeyPress(ui *uiterm.Ui, tree *uiterm.Tree, item uiterm
 		if key == *b.Hotkeys.MuteToggle {
 			// Toggle mute for single user
 			b.UserConfig.ToggleMute(treeItem.User)
-			if treeItem.User.AudioSource != nil {
-				if treeItem.User.LocallyMuted {
-					treeItem.User.AudioSource.SetGain(0)
+			if au := treeItem.User.AudioSource(); au != nil {
+				if treeItem.User.LocallyMuted() {
+					au.SetGain(0)
 				} else {
-					treeItem.User.AudioSource.SetGain(treeItem.User.Volume)
+					au.SetGain(treeItem.User.Volume())
 				}
 			}
 			b.RebuildUserChannelTreePreservingSelection()