swarm-ui: dropdowns open a themed list of their own instead of the browser's

SelectField and the refresh-interval picker are built on a new Listbox, a
select-only combobox: a button that keeps focus and a listbox in the top
layer, so it works the same in every current browser, is not clipped by a
scrolling ancestor and shows above a modal dialog. Keys, type-ahead and
placement are pure functions with tests. required becomes aria-required, as a
button has no native form validation.

frontend/packages/swarm-ui/src/pages/CreateAgentForm.tsx

@@ -14,7 +14,7 @@
 //
 // `hive` field added because agent creation had no way to record which
 // hive an agent runs on. `POST /api/agents` now requires it, so the
-// roster fetched off `GET /api/hives` backs a required select here
+// roster fetched off `GET /api/hives` backs a required dropdown here
 // rather than a free-text field — a spawn target has to be one of the
 // swarm's actual hives, same reasoning `Ident.parse` gets client-side
 // pattern validation for `name`.
@@ -159,8 +159,8 @@ export function CreateAgentForm({
       setHives(data);
       // Single-hive swarms are the common case — default it rather than
       // making the operator pick the only option. Multi-hive swarms get
-      // no default (the placeholder stays selected), so `required` on
-      // the select below forces an explicit choice per mara's scope.
+      // no default (the placeholder stays selected), so the identity step
+      // cannot be left until one is picked (`stepProblem`), per mara's scope.
       if (data.length === 1) setHive(data[0].name);
     })().catch((e: unknown) => setHivesError({ detail: String(e) }));
   }, []);