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>

twilio-wp-plugin.php

@@ -15,7 +15,7 @@ if (!defined('WPINC')) {
 }
 
 // Plugin constants
-define('TWP_VERSION', '1.3.13');
+define('TWP_VERSION', '2.1.0');
 define('TWP_DB_VERSION', '1.1.0'); // Track database version separately
 define('TWP_PLUGIN_DIR', plugin_dir_path(__FILE__));
 define('TWP_PLUGIN_URL', plugin_dir_url(__FILE__));