[FIX] web: ok mobile kanban w/o o_kanban_small_column

On mobile, kanban view always have its custom "menu" bar to cycle
between grouping if the kanban view is grouped.

But if the kanban view did not have the class `o_kanban_small_column`
the mobile CSS on record was not applied:

- all columns were always displayed one over the other
- switching tab did not hides other columns and show current one
- the style was the same than desktop (but on mobile and with column
  after first "to be loaded)

opw-1827142
closes #23691

addons/web/static/src/less/kanban_view_mobile.less

@@ -29,7 +29,8 @@
             }
         }
 
-        &.o_kanban_small_column .o_kanban_group:not(.o_column_folded) {
+        // [class] to get same specificity as elsewhere (kanban_view.less)
+        &[class] .o_kanban_group:not(.o_column_folded) {
             .o-position-absolute(@top: @o-kanban-mobile-tabs-height, @left: 0, @bottom: 0);
             overflow-x: scroll;
             width: 100%;