Add F12 hotkey to toggle microphone AGC

Automatic gain control was always on with no way to turn it off. Toggle
it with F12, the /agc command, or the agc FIFO command, and persist the
choice in the configuration file the same way noise suppression does.

AgcEnabled defaults to true so existing setups keep their current
behavior, and the saved value is applied to the stream on connect. The
enabled flag becomes an atomic.Bool because the capture goroutine reads
it while the UI goroutine writes it, and the lazily created right
channel AGC now inherits the left channel's state.

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

client.go

@@ -90,6 +90,7 @@ func (b *Barnard) connect(reconnect bool) bool {
 	stream.SetMicVolume(b.UserConfig.GetMicVolume(), false)
 	stream.AttachStream(b.Client)
 	stream.SetNoiseProcessor(b.NoiseSuppressor)
+	stream.SetAGCEnabled(b.UserConfig.GetAGCEnabled())
 	stream.SetErrorFunc(func(err error) {
 		if err != nil {
 			b.AddOutputLine(fmt.Sprintf("Microphone: %s", err.Error()))