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>

config/hotkey_config.go

@@ -19,4 +19,5 @@ type Hotkeys struct {
     ScrollToTop      *uiterm.Key
     ScrollToBottom   *uiterm.Key
     NoiseSuppressionToggle *uiterm.Key
+    CycleVoiceEffect *uiterm.Key
 }

config/user_config.go

@@ -28,6 +28,7 @@ type exportableConfig struct {
     NotifyCommand *string
     NoiseSuppressionEnabled    *bool
     NoiseSuppressionThreshold *float32
+    VoiceEffect   *int
 }
 
 type server struct {
@@ -78,6 +79,7 @@ func (c *Config) LoadConfig() {
         ScrollUp:         key(uiterm.KeyPgup),
         ScrollDown:       key(uiterm.KeyPgdn),
         NoiseSuppressionToggle: key(uiterm.KeyF9),
+        CycleVoiceEffect: key(uiterm.KeyF12),
     }
     if fileExists(c.fn) {
         var data []byte
@@ -123,6 +125,10 @@ func (c *Config) LoadConfig() {
         threshold := float32(0.02)
         jc.NoiseSuppressionThreshold = &threshold
     }
+    if c.config.VoiceEffect == nil {
+        effect := 0 // Default to EffectNone
+        jc.VoiceEffect = &effect
+    }
 }
 
 func (c *Config) findServer(address string) *server {
@@ -232,6 +238,18 @@ func (c *Config) SetNoiseSuppressionThreshold(threshold float32) {
     c.SaveConfig()
 }
 
+func (c *Config) GetVoiceEffect() int {
+    if c.config.VoiceEffect == nil {
+        return 0
+    }
+    return *c.config.VoiceEffect
+}
+
+func (c *Config) SetVoiceEffect(effect int) {
+    c.config.VoiceEffect = &effect
+    c.SaveConfig()
+}
+
 func (c *Config) UpdateUser(u *gumble.User) {
     var j *eUser
     var uc *gumble.Client