make microphone AGC less aggressive and switchable

Lower the gain ceiling and slow the attack of the automatic gain
control.
It amplified room noise between words hard enough to be audible as a
rising hiss whenever the speaker paused.

Add an AGC toggle on F12 and an /agc command.
AGC has always been applied unconditionally, which is wrong for a
microphone that is already levelled by hardware or by the system
mixer. The preference is saved and reapplied on connect, defaulting to
on so existing setups are unaffected.

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

client.go

@@ -53,6 +53,7 @@ func (b *Barnard) connect(reconnect bool) bool {
 	b.Stream = stream
 	b.Stream.AttachStream(b.Client)
 	b.Stream.SetNoiseProcessor(b.NoiseSuppressor)
+	b.Stream.SetAGCEnabled(b.UserConfig.GetAGCEnabled())
 
 	// Initialize stereo encoder for file playback
 	b.Client.AudioEncoderStereo = opus.NewStereoEncoder()