Fix extra button to only work when OFF-hook and add greeting delay

Breaking Behavior Change:
- Extra button NOW only works when handset is OFF-hook (during call)
- Previous: worked when on-hook (idle) - REVERSED
- Reason: Button is for guests to trigger during their call/recording

New Feature - Greeting Delay:
- Add greeting_delay_seconds to config.json (0-10 seconds)
- Delays greeting playback after handset pickup
- Gives guests time to position phone comfortably
- Default: 0 (plays immediately, backward compatible)

Extra Button Logic:
- Only responds when phone_status == "off_hook"
- Ignored when phone is on-hook (idle)
- Can be pressed during greeting or recording
- Plays entire sound without interruption
- Useful for: "Press button to hear special message"

Greeting Delay Use Cases:
- Wedding: Give guests moment to settle in
- Events: Time to position phone comfortably
- Accessibility: Extra time for elderly guests
- Professional: Pause before message delivery

Configuration:
{
  "system": {
    "greeting_delay_seconds": 2,  // 2 second pause
    "extra_button_sound": "surprise.wav"
  }
}

Console Output:
- "Extra button ignored - phone is on hook" (when idle)
- "Waiting X seconds before greeting..." (delay active)
- "=== Extra button pressed ===" (during call)

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

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

README.md

@@ -189,10 +189,21 @@ The web interface provides four main sections:
 ### 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
+1. **Guest picks up phone**: Phone goes off-hook
+2. **Guest presses button**: System detects GPIO signal (only works when off-hook)
+3. **Sound plays**: Configured button sound plays through speaker
+4. **Can be pressed multiple times**: Works during greeting or recording
+5. **Debounced**: 0.5s delay prevents accidental double-presses
+
+**Note:** Button only responds when handset is off-hook (during active call)
+
+### Greeting Delay (Optional)
+
+Configure a delay before the greeting plays:
+- Set `greeting_delay_seconds` in `config.json` (0-10 seconds)
+- Useful for giving guests a moment after picking up
+- Example: 2 second delay gives time to position the phone
+- Default: 0 (greeting plays immediately)
 
 ## File Structure
 
@@ -256,6 +267,7 @@ The `config.json` file contains all system settings:
   "system": {
     "active_greeting": "dialtone.wav",     // Default greeting
     "extra_button_sound": "button_sound.wav", // Default button sound
+    "greeting_delay_seconds": 0,           // Delay before greeting plays (0-10)
     "volume": 70                           // Default volume (0-100)
   }
 }