Add default queue music setting and improve hold music fallback

## New Settings
- Added "Default Queue Music URL" setting in admin settings
- This setting provides fallback music for both queue waits and call hold
- Hold Music URL field can now be left empty to use default queue music
- Better organization of music settings with clear descriptions

## Hold Music Improvements
- Hold music now falls back to default queue music if no specific hold music is set
- Maintains backwards compatibility with existing hold music URLs
- Provides consistent music experience across hold and queue functions

## Queue Music Enhancements
- Queue wait music now uses default queue music as fallback
- No more silent pauses when no queue-specific music is configured
- Improved user experience with continuous music during waits

## Settings Organization
- Default Queue Music is shown first as the primary setting
- Hold Music field has placeholder text indicating it's optional
- Clear descriptions explain the fallback behavior
- Both settings properly registered in WordPress settings API

This creates a more cohesive audio experience where:
1. Admin sets default queue music for system-wide use
2. Individual queues can override with their own music
3. Hold music can be customized or fall back to default
4. No more silent pauses during queue waits

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

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

includes/class-twp-webhooks.php

@@ -973,8 +973,14 @@ class TWP_Webhooks {
                 if ($queue && !empty($queue->wait_music_url)) {
                     $play = $twiml->addChild('Play', $queue->wait_music_url);
                 } else {
-                    $pause = $twiml->addChild('Pause');
-                    $pause->addAttribute('length', '30');
+                    // Fall back to default queue music if no queue-specific music is set
+                    $default_music_url = get_option('twp_default_queue_music_url', '');
+                    if (!empty($default_music_url)) {
+                        $play = $twiml->addChild('Play', $default_music_url);
+                    } else {
+                        $pause = $twiml->addChild('Pause');
+                        $pause->addAttribute('length', '30');
+                    }
                 }
                 
                 return $this->send_twiml_response($twiml->asXML());
@@ -996,9 +1002,15 @@ class TWP_Webhooks {
             if ($queue && !empty($queue->wait_music_url)) {
                 $play = $twiml->addChild('Play', $queue->wait_music_url);
             } else {
-                // Add a pause to prevent rapid loops
-                $pause = $twiml->addChild('Pause');
-                $pause->addAttribute('length', '15'); // 15 second pause
+                // Fall back to default queue music if no queue-specific music is set
+                $default_music_url = get_option('twp_default_queue_music_url', '');
+                if (!empty($default_music_url)) {
+                    $play = $twiml->addChild('Play', $default_music_url);
+                } else {
+                    // Add a pause to prevent rapid loops
+                    $pause = $twiml->addChild('Pause');
+                    $pause->addAttribute('length', '15'); // 15 second pause
+                }
             }
             
             // Redirect back to this same endpoint to create continuous loop