make per-user audio state thread-safe

Move volume, boost, mute, and OpenAL source behind accessors guarded
by a per-user mutex.
The audio goroutine reads these fields on every decoded packet while
the UI writes them from key handlers, which is a data race on the
gain a stream is currently rendering with.

Also add the per-user sequence fields the decoder needs to notice
gaps in a user's audio stream.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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 source := e.User.AudioSource(); source != nil {
+					source.SetGain(0)
 				}
 			}
 		}