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>

client.go

@@ -50,6 +50,7 @@ func (b *Barnard) connect(reconnect bool) bool {
     b.Stream = stream
     b.Stream.AttachStream(b.Client)
     b.Stream.SetNoiseProcessor(b.NoiseSuppressor)
+    b.Stream.SetEffectsProcessor(b.VoiceEffects)
     b.Connected = true
     return true
 }