Add IntOrFalse type to handle polymorphic API responses

Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.

tags.go

@@ -75,11 +75,11 @@ func (c *Client) CreateTag(ctx context.Context, projectID int, name, colorID str
 		params["color_id"] = colorID
 	}
 
-	var result int
+	var result IntOrFalse
 	if err := c.call(ctx, "createTag", params, &result); err != nil {
 		return 0, fmt.Errorf("createTag: %w", err)
 	}
-	return result, nil
+	return int(result), nil
 }
 
 // UpdateTag updates an existing tag's name and/or color.