Add extra GPIO button support with custom sound playback

Features:
- Configurable extra button on any GPIO pin
- Upload and select custom sound for button press
- Non-blocking button operation (separate thread)
- Debounced to prevent double-triggers (0.5s)
- Works independently from phone handset
- Can be pressed anytime, even during recording

Configuration:
- gpio.extra_button_enabled: Enable/disable feature
- gpio.extra_button_pin: GPIO pin number (default: 27)
- gpio.extra_button_pressed_state: "LOW" or "HIGH"
- system.extra_button_sound: Default sound file

Web Interface:
- Display active button sound in alert
- "🔘 Set Button" action on greeting items
- Visual indicator (🔘) for active button sound
- Upload any WAV file and assign to button
- Play/preview button sounds in browser

Backend:
- RotaryPhone.play_extra_button_sound() method
- RotaryPhone.set_extra_button_sound() method
- Thread-based playback to not block main loop
- /set_extra_button_sound API endpoint
- Extra button sound tracked in user_config.json

Documentation:
- Extra button setup in README
- Configuration examples
- GPIO pin configuration
- Operation workflow

Use Cases:
- Play special message on button press
- Sound effects for wedding games
- Multiple interaction points
- Custom audio triggers

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

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

README.md

@@ -12,6 +12,7 @@ A Raspberry Pi-based rotary phone system for weddings and events. Guests can pic
 - **Volume Control**: Adjust playback volume with real-time slider (0-100%)
 - **Multiple Message Support**: Upload and manage multiple greeting messages
 - **Active Message Selector**: Choose which greeting plays when the phone is picked up
+- **Extra Button Support**: Optional GPIO button to play custom sounds on demand
 - **HiFiBerry Support**: Optimized for HiFiBerry DAC+ADC Pro audio quality
 - **Real-time Status**: Monitor phone status (on-hook/off-hook/recording)
 - **Auto-refresh**: Status updates every 5 seconds
@@ -100,8 +101,11 @@ nano config.json  # or use your preferred editor
 ```json
 {
   "gpio": {
-    "hook_pin": 17,                    // GPIO pin for hookswitch
-    "hook_pressed_state": "LOW"        // "LOW" or "HIGH"
+    "hook_pin": 17,                        // GPIO pin for hookswitch
+    "hook_pressed_state": "LOW",           // "LOW" or "HIGH"
+    "extra_button_enabled": true,          // Enable extra button feature
+    "extra_button_pin": 27,                // GPIO pin for extra button
+    "extra_button_pressed_state": "LOW"    // "LOW" or "HIGH"
   },
   "audio": {
     "device_index": 1,                 // HiFiBerry device index
@@ -164,6 +168,7 @@ The web interface provides four main sections:
 - **Upload**: Click "Choose WAV File(s)" to upload one or multiple greeting messages
 - **Play**: Click "▶️ Play" to preview any greeting in your browser
 - **Set Active**: Click "⭐ Set Active" to select which greeting plays when the phone is picked up
+- **Set Button**: Click "🔘 Set Button" to assign sound to extra button (if enabled)
 - **Delete**: Remove unwanted greetings (cannot delete the active one)
 - **Default Tone**: Generate a classic telephone dial tone
 
@@ -181,6 +186,14 @@ The web interface provides four main sections:
 4. **Guest hangs up**: Recording stops and saves automatically
 5. **Ready for next call**: System returns to waiting state
 
+### Extra Button Operation (Optional)
+
+If enabled in `config.json`:
+1. **Guest presses button**: System detects GPIO signal
+2. **Sound plays**: Configured button sound plays through speaker
+3. **Non-blocking**: Button can be pressed anytime, even during recording
+4. **Debounced**: 0.5s delay prevents accidental double-presses
+
 ## File Structure
 
 ```
@@ -218,7 +231,10 @@ The `config.json` file contains all system settings:
 {
   "gpio": {
     "hook_pin": 17,                        // GPIO pin number for hookswitch
-    "hook_pressed_state": "LOW"            // "LOW" or "HIGH" depending on switch
+    "hook_pressed_state": "LOW",           // "LOW" or "HIGH" depending on switch
+    "extra_button_enabled": true,          // Enable optional extra button
+    "extra_button_pin": 27,                // GPIO pin for extra button
+    "extra_button_pressed_state": "LOW"    // Button pressed state
   },
   "audio": {
     "device_index": 1,                     // Audio device index (run test to find)
@@ -239,6 +255,7 @@ The `config.json` file contains all system settings:
   },
   "system": {
     "active_greeting": "dialtone.wav",     // Default greeting
+    "extra_button_sound": "button_sound.wav", // Default button sound
     "volume": 70                           // Default volume (0-100)
   }
 }