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>

barnard.go

@@ -3,6 +3,7 @@ package main
 import (
     "crypto/tls"
 
+    "git.stormux.org/storm/barnard/audio"
     "git.stormux.org/storm/barnard/config"
     "git.stormux.org/storm/barnard/gumble/gumble"
     "git.stormux.org/storm/barnard/gumble/gumbleopenal"
@@ -44,9 +45,12 @@ type Barnard struct {
 
     // Added for channel muting
     MutedChannels map[uint32]bool
-    
+
     // Added for noise suppression
     NoiseSuppressor *noise.Suppressor
+
+    // Added for voice effects
+    VoiceEffects *audio.EffectsProcessor
 }
 
 func (b *Barnard) StopTransmission() {