feat(audio): optimize equalizer with stereo support and gain caching

pkg/audio/fft.go

@@ -83,7 +83,7 @@ func CalculateEQBands(samples []int16, sampleRate int) []float64 {
 
 	// Normalize output for visualization (0.0 to 1.0)
 	// We need some scaling factor. Based on expected signals.
-	const scale = 50.0 // heuristic
+	const scale = 10.0 // Reduced from 50.0 to fix saturation
 	for i := range bands {
 		bands[i] = bands[i] * scale
 		if bands[i] > 1.0 {