Change extra button to only work during recording phase

- Extra button now only responds during actual recording
- Does not work during greeting playback or when on-hook
- Updated logic from checking off-hook status to checking recording status
- Updated README to reflect recording-only behavior

This allows guests to add sound effects to their recorded message
without interrupting the initial greeting.

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

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

rotary_phone_web.py

@@ -319,13 +319,13 @@ class RotaryPhone:
         }
     
     def play_extra_button_sound(self):
-        """Play sound when extra button is pressed (only when off-hook/in call)"""
+        """Play sound when extra button is pressed (only during recording)"""
         if not EXTRA_BUTTON_ENABLED:
             return
 
-        # Only play if phone is OFF hook (during a call)
-        if self.phone_status != "off_hook":
-            print("Extra button ignored - phone is on hook")
+        # Only play if currently recording
+        if not self.recording:
+            print("Extra button ignored - not recording")
             return
 
         button_sound = self.get_extra_button_sound_path()