feat: support ALTER PAGE SET Url for changing page URLs

The grammar already parsed SET Url = '...' via the generic page-level
SET rule but the executor rejected it as unsupported. Added the Url
case to applyPageLevelSet — stored as a plain string in BSON.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

mdl/executor/cmd_alter_page.go

@@ -569,6 +569,10 @@ func applyPageLevelSet(rawData bson.D, properties map[string]interface{}) error
 			} else {
 				setTranslatableText(rawData, "Title", value)
 			}
+		case "Url":
+			// URL is stored as a plain string at the top level
+			strVal, _ := value.(string)
+			dSet(rawData, "Url", strVal)
 		default:
 			return fmt.Errorf("unsupported page-level property: %s", propName)
 		}