Give quick-add rows their listbox semantics

Create rows inside the combobox dropdown now carry role="option" like
their sibling matches, and a failed creation announces itself with
role="alert" instead of a silent list item.

web/src/ui.tsx

@@ -227,6 +227,8 @@ export function Combobox({
               <button
                 type="button"
                 className="combo-option create"
+                role="option"
+                aria-selected={false}
                 disabled={creating}
                 onMouseDown={(e) => e.preventDefault()}
                 onClick={() => void runCreate(c)}
@@ -236,7 +238,11 @@ export function Combobox({
               </button>
             </li>
           ))}
-          {createError && <li className="combo-empty error">{createError}</li>}
+          {createError && (
+            <li className="combo-empty error" role="alert">
+              {createError}
+            </li>
+          )}
           {shown.length === 0 && creations.length === 0 && !createError && (
             <li className="combo-empty">{emptyText}</li>
           )}