Give the map editor's search box room for its own placeholder

The previous commit stopped the panel header from colliding with the "Assets"
heading, but left the search box sharing a row with the type dropdown: 139px of
input for a placeholder needing 216px, so it still read "Search name o". A
control whose own label does not fit is not a narrower control, it is an
unlabelled one.

One control per row in a 318px panel. Measured in the browser: the input is now
286px against 216px of text, so the placeholder reads in full.

frontend/src/views/MapEditor.vue

@@ -534,8 +534,11 @@ function cancelEdit() {
 }
 
 .panel-header .form-control {
-  /* Shrinkable and equal, so nothing gets crushed to fit a sibling. */
-  flex: 1 1 8rem;
+  /* One control per row. Two of them side by side in a 318px panel left the
+     search box 139px for a placeholder needing 216px, so it read "Search name
+     o" - a control whose own label does not fit is not a narrower control, it is
+     an unlabelled one. */
+  flex: 1 1 100%;
   min-width: 0;
 }