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>

audio/agc.go

@@ -2,41 +2,43 @@ package audio
 
 import (
 	"math"
+	"sync/atomic"
 )
 
 // AGC (Automatic Gain Control) processor for voice normalization
 type AGC struct {
-	targetLevel    float32 // Target RMS level (0.0-1.0)
-	maxGain        float32 // Maximum gain multiplier
-	minGain        float32 // Minimum gain multiplier
-	attackTime     float32 // Attack time coefficient
-	releaseTime    float32 // Release time coefficient
-	currentGain    float32 // Current gain value
-	envelope       float32 // Signal envelope
-	enabled        bool    // Whether AGC is enabled
-	compThreshold  float32 // Compression threshold
-	compRatio      float32 // Compression ratio
+	targetLevel   float32     // Target RMS level (0.0-1.0)
+	maxGain       float32     // Maximum gain multiplier
+	minGain       float32     // Minimum gain multiplier
+	attackTime    float32     // Attack time coefficient
+	releaseTime   float32     // Release time coefficient
+	currentGain   float32     // Current gain value
+	envelope      float32     // Signal envelope
+	enabled       atomic.Bool // Whether AGC is enabled; toggled outside the capture goroutine
+	compThreshold float32     // Compression threshold
+	compRatio     float32     // Compression ratio
 }
 
 // NewAGC creates a new AGC processor with sensible defaults for voice
 func NewAGC() *AGC {
-	return &AGC{
-		targetLevel:    0.12,  // Target 12% of max amplitude (conservative level)
-		maxGain:        4.0,   // Maximum 4x gain (about 12dB)
-		minGain:        0.25,  // Minimum 0.25x gain (-12dB)
-		attackTime:     0.008, // Fast attack (8ms)
-		releaseTime:    0.15,  // Slower release (150ms)
-		currentGain:    1.0,   // Start with unity gain
-		envelope:       0.0,   // Start with zero envelope
-		enabled:        true,  // Enable by default
-		compThreshold:  0.85,  // Compress signals above 85%
-		compRatio:      2.0,   // 2:1 compression ratio (gentler)
+	agc := &AGC{
+		targetLevel:   0.12,  // Target 12% of max amplitude (conservative level)
+		maxGain:       4.0,   // Maximum 4x gain (about 12dB)
+		minGain:       0.25,  // Minimum 0.25x gain (-12dB)
+		attackTime:    0.008, // Fast attack (8ms)
+		releaseTime:   0.15,  // Slower release (150ms)
+		currentGain:   1.0,   // Start with unity gain
+		envelope:      0.0,   // Start with zero envelope
+		compThreshold: 0.85,  // Compress signals above 85%
+		compRatio:     2.0,   // 2:1 compression ratio (gentler)
 	}
+	agc.enabled.Store(true) // Enable by default
+	return agc
 }
 
 // ProcessSamples applies AGC processing to audio samples
 func (agc *AGC) ProcessSamples(samples []int16) {
-	if !agc.enabled || len(samples) == 0 {
+	if !agc.enabled.Load() || len(samples) == 0 {
 		return
 	}
 
@@ -125,12 +127,12 @@ func (agc *AGC) ProcessSamples(samples []int16) {
 
 // SetEnabled enables or disables AGC processing
 func (agc *AGC) SetEnabled(enabled bool) {
-	agc.enabled = enabled
+	agc.enabled.Store(enabled)
 }
 
 // IsEnabled returns whether AGC is enabled
 func (agc *AGC) IsEnabled() bool {
-	return agc.enabled
+	return agc.enabled.Load()
 }
 
 // SetTargetLevel sets the target RMS level (0.0-1.0)