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>

config/user_config.go

@@ -31,6 +31,7 @@ type exportableConfig struct {
 	Username                *string
 	NotifyCommand           *string
 	NoiseSuppressionEnabled *bool
+	AGCEnabled              *bool
 	Certificate             *string
 	RecordingFormat         *string
 	RecordingDirectory      *string
@@ -109,6 +110,7 @@ func (c *Config) LoadConfig() {
 		ScrollToBottom:         key(uiterm.KeyEnd),
 		AdminMenu:              key(uiterm.KeyF11),
 		NoiseSuppressionToggle: key(uiterm.KeyF9),
+		AGCToggle:              key(uiterm.KeyF12),
 	}
 	if fileExists(c.fn) {
 		var data []byte
@@ -155,6 +157,11 @@ func (c *Config) LoadConfig() {
 		enabled := false
 		jc.NoiseSuppressionEnabled = &enabled
 	}
+	if c.config.AGCEnabled == nil {
+		// AGC has always been active for the microphone, so keep it on by default.
+		enabled := true
+		jc.AGCEnabled = &enabled
+	}
 	if c.config.Certificate == nil {
 		cert := string("")
 		jc.Certificate = &cert
@@ -190,6 +197,7 @@ func (c *Config) ensureHotkeys() {
 		ScrollToBottom:         key(uiterm.KeyEnd),
 		AdminMenu:              key(uiterm.KeyF11),
 		NoiseSuppressionToggle: key(uiterm.KeyF9),
+		AGCToggle:              key(uiterm.KeyF12),
 	}
 	hotkeys := c.config.Hotkeys
 	if hotkeys.Talk == nil {
@@ -240,6 +248,9 @@ func (c *Config) ensureHotkeys() {
 	if hotkeys.NoiseSuppressionToggle == nil {
 		hotkeys.NoiseSuppressionToggle = defaults.NoiseSuppressionToggle
 	}
+	if hotkeys.AGCToggle == nil {
+		hotkeys.AGCToggle = defaults.AGCToggle
+	}
 }
 
 func (c *Config) findServer(address string) *server {
@@ -365,6 +376,22 @@ func (c *Config) SetNoiseSuppressionEnabled(enabled bool) error {
 	return c.saveConfigLocked()
 }
 
+func (c *Config) GetAGCEnabled() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.config.AGCEnabled == nil {
+		return true
+	}
+	return *c.config.AGCEnabled
+}
+
+func (c *Config) SetAGCEnabled(enabled bool) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.config.AGCEnabled = &enabled
+	return c.saveConfigLocked()
+}
+
 func (c *Config) GetRecordingFormat() string {
 	c.mu.Lock()
 	defer c.mu.Unlock()