Add real-time voice effects for outgoing audio

Implements 7 voice effects that can be cycled through with F12:
- None (default)
- Echo: Single repeating delay with feedback (250ms)
- Reverb: Multiple short delays without feedback
- High Pitch: Chipmunk voice using cubic interpolation
- Low Pitch: Deep voice effect
- Robot: Ring modulation for robotic sound
- Chorus: Layered voices with pitch variations

The effects are applied after noise suppression and AGC in the audio
pipeline. Selected effect is persisted to config file. Includes
comprehensive documentation in README.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

main.go

@@ -16,6 +16,7 @@ import (
 	"crypto/tls"
 	"flag"
 	"github.com/alessio/shellescape"
+	"git.stormux.org/storm/barnard/audio"
 	"git.stormux.org/storm/barnard/config"
 	"git.stormux.org/storm/barnard/noise"
 
@@ -162,6 +163,7 @@ func main() {
 		Address:    *server,
 		MutedChannels: make(map[uint32]bool),
 		NoiseSuppressor: noise.NewSuppressor(),
+		VoiceEffects: audio.NewEffectsProcessor(gumble.AudioSampleRate),
 	}
 	b.Config.Buffers = *buffers
 
@@ -176,7 +178,10 @@ func main() {
 	}
 	b.NoiseSuppressor.SetEnabled(enabled)
 	b.NoiseSuppressor.SetThreshold(b.UserConfig.GetNoiseSuppressionThreshold())
-	
+
+	// Configure voice effects
+	b.VoiceEffects.SetEffect(audio.VoiceEffect(b.UserConfig.GetVoiceEffect()))
+
 	b.Config.Username = *username
 	b.Config.Password = *password