refactor: use data_col_to_visual via group_for helpers, add column group toggle

Add row_group_for/col_group_for to GridLayout, replacing inline
backward-search logic. Refactor grid renderer to use col_group_for
instead of pre-filtering col_items. Add gz keybinding for column
group collapse toggle, symmetric with z for rows.

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

src/model/category.rs

@@ -164,8 +164,7 @@ mod tests {
         assert_eq!(c.groups.len(), 1);
     }
 
-
-#[test]
+    #[test]
     fn item_index_reflects_insertion_order() {
         let mut c = cat();
         c.add_item("East");