Fix validation error for multiple phone numbers

Updated workflow save validation to check for phone_numbers[] array
instead of the old single phone_number field. Now properly validates
that at least one phone number is selected before allowing save.

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

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

assets/js/admin.js

@@ -1094,8 +1094,17 @@ jQuery(document).ready(function($) {
             return;
         }
         
-        if (!workflowData.phone_number) {
-            alert('Please select a phone number');
+        // Check if at least one phone number is selected
+        var hasPhoneNumbers = false;
+        $('#workflow-phone-numbers select[name="phone_numbers[]"]').each(function() {
+            if ($(this).val()) {
+                hasPhoneNumbers = true;
+                return false; // break out of loop
+            }
+        });
+        
+        if (!hasPhoneNumbers) {
+            alert('Please select at least one phone number');
             return;
         }