Add mobile-friendly browser phone shortcode for frontend use

- Created TWP_Shortcodes class with [twp_browser_phone] shortcode
- Mobile-first responsive CSS design with touch-friendly interface
- Frontend JavaScript integration with Twilio Voice SDK v2
- Permission checks for logged-in users with twp_access_browser_phone capability
- Haptic feedback and dark mode support
- Configurable shortcode parameters (title, show_title, compact)
- Updated plugin version to 2.1.0 in main file
- Added comprehensive documentation to README.md

Features:
- Visual dialpad with DTMF support
- Real-time connection status indicators
- Call timer and queue management
- Auto-scaling for mobile devices (320px+)
- Optimized for both desktop and mobile use

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

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

includes/class-twp-core.php

@@ -43,6 +43,7 @@ class TWP_Core {
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-agent-groups.php';
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-agent-manager.php';
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-callback-manager.php';
+        require_once TWP_PLUGIN_DIR . 'includes/class-twp-shortcodes.php';
         
         // Admin classes
         require_once TWP_PLUGIN_DIR . 'admin/class-twp-admin.php';
@@ -197,6 +198,9 @@ class TWP_Core {
         $this->loader->add_action('wp_ajax_twp_delete_conversation', $plugin_admin, 'ajax_delete_conversation');
         $this->loader->add_action('wp_ajax_twp_get_conversation', $plugin_admin, 'ajax_get_conversation');
         $this->loader->add_action('wp_ajax_twp_send_sms_reply', $plugin_admin, 'ajax_send_sms_reply');
+        
+        // Frontend browser phone AJAX handlers are already covered by the admin handlers above
+        // since they check permissions internally
     }
     
     /**
@@ -210,6 +214,9 @@ class TWP_Core {
         // Initialize Agent Manager
         TWP_Agent_Manager::init();
         
+        // Initialize Shortcodes
+        TWP_Shortcodes::init();
+        
         // Scheduled events
         $scheduler = new TWP_Scheduler();
         $this->loader->add_action('twp_check_schedules', $scheduler, 'check_active_schedules');