Fix browser phone permission issues for phone agents

- Fixed error message display when browser phone initialization fails (now shows actual error instead of "undefined")
- Fixed ajax_get_phone_numbers to use wp_send_json_error instead of wp_die for proper error handling
- Added error handling for phone numbers loading in browser phone interface
- Updated phone_agent role to ensure all necessary capabilities are added to existing roles
- Added automatic capability updates for existing phone_agent roles on plugin load

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

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

includes/class-twp-core.php

@@ -89,8 +89,20 @@ class TWP_Core {
             ));
         } else {
             // Update existing role with any missing capabilities
-            if (!$role->has_cap('twp_access_phone_numbers')) {
-                $role->add_cap('twp_access_phone_numbers');
+            $capabilities = array(
+                'twp_access_voicemails',
+                'twp_access_call_log',
+                'twp_access_agent_queue',
+                'twp_access_outbound_calls',
+                'twp_access_sms_inbox',
+                'twp_access_browser_phone',
+                'twp_access_phone_numbers'
+            );
+            
+            foreach ($capabilities as $cap) {
+                if (!$role->has_cap($cap)) {
+                    $role->add_cap($cap);
+                }
             }
         }
     }