Arrow-key navigation for the combobox

Arrow keys cycle through the listed matches and create rows with
aria-activedescendant tracking, and Enter activates the armed row.
This closes a keyboard-only gap in quick-add: with matches still
listed, Enter deliberately refuses to mint from a half-typed name,
which left the create row reachable only by mouse.

web/src/styles.css

@@ -2076,6 +2076,7 @@ footer span:first-child {
   color: inherit;
 }
 .combo-option:hover,
+.combo-option.active,
 .combo-option[aria-selected="true"] {
   background: #f0f7f4;
 }