[DREAM-801] Reserve control height in list headers

Headers whose action buttons are hidden while filtering shrank by
the button height. The heading line now reserves the medium control
height, replacing the caller-side title padding the migrations
dropped.

https://community.openproject.org/wp/DREAM-801

Author: myabc

app/components/open_project/common/border_box_list_component.sass

@@ -154,6 +154,9 @@
     display: flex
     align-items: center
     gap: var(--stack-gap-condensed)
+    // Reserve the height of a medium control so headers whose action
+    // buttons toggle (e.g. hidden while filtering) keep a stable height.
+    min-height: var(--control-medium-size)
 
     .Box-title
       min-width: 0