Fix critical browser phone shortcode issues

Issues Fixed:
1. 🎵 Audio Error: Removed missing ringtone.mp3 causing 404 error
2. 📡 Twilio SDK Loading: Added proper SDK availability checking and retry mechanism
3. 🎨 Contrast Issues: Improved readability with darker text colors

Technical Changes:
- Removed hardcoded ringtone.mp3 reference causing 404 errors
- Added waitForTwilioSDK() function with 5-second timeout and retry logic
- Changed Twilio SDK loading to head (false flag) for better availability
- Enhanced error handling with specific Twilio SDK load failure messages
- Improved text contrast: #6c757d → #495057, #495057 → #212529
- Enhanced dark mode colors for better accessibility

UI Improvements:
- Status text: font-weight 600, color #212529 for better visibility
- Labels and selects: improved contrast ratios
- Queue info: better text color for mobile readability
- Call status: font-weight 600 for prominence
- Dark mode: complete color scheme overhaul with proper contrast

Error Handling:
- Graceful SDK loading with visual feedback
- Clear error messages for troubleshooting
- Automatic retry mechanism for network issues
- Timeout handling for slow connections

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

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

includes/class-twp-shortcodes.php

@@ -32,7 +32,7 @@ class TWP_Shortcodes {
                 'https://sdk.twilio.com/js/voice/2.11.1/twilio.min.js',
                 array(),
                 '2.11.1',
-                true
+                false // Load in head to ensure it's available
             );
             
             // Enqueue our browser phone script
@@ -180,10 +180,8 @@ class TWP_Shortcodes {
                 </div>
             </div>
             
-            <!-- Audio elements -->
-            <audio id="twp-ringtone" preload="auto" loop>
-                <source src="<?php echo TWP_PLUGIN_URL; ?>assets/audio/ringtone.mp3" type="audio/mpeg">
-            </audio>
+            <!-- Audio elements for incoming calls handled by browser -->
+            <audio id="twp-ringtone" style="display: none;"></audio>
             
             <!-- Error/Success Messages -->
             <div class="twp-messages" id="twp-messages"></div>