Add comprehensive voicemail integration to browser phone

- Add AJAX endpoint for fetching user voicemails with stats and recent list
- Create voicemail display section with counts, transcriptions, and playback
- Implement click-to-play functionality for voicemail audio
- Add refresh and view-all buttons with admin page integration
- Style voicemail section with consistent design and dark mode support
- Include visual indicators for recordings and formatted durations
- Add mobile responsive design and hover effects

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

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

includes/class-twp-core.php

@@ -156,6 +156,7 @@ class TWP_Core {
         $this->loader->add_action('wp_ajax_twp_delete_voicemail', $plugin_admin, 'ajax_delete_voicemail');
         $this->loader->add_action('wp_ajax_twp_transcribe_voicemail', $plugin_admin, 'ajax_transcribe_voicemail');
         $this->loader->add_action('wp_ajax_twp_get_voicemail_audio', $plugin_admin, 'ajax_get_voicemail_audio');
+        $this->loader->add_action('wp_ajax_twp_get_user_voicemails', $plugin_admin, 'ajax_get_user_voicemails');
         
         // Agent group management AJAX
         $this->loader->add_action('wp_ajax_twp_get_all_groups', $plugin_admin, 'ajax_get_all_groups');