swarm-ui: show, sort and filter the hive domain by one value, say when the roster is empty, keep the schedule agent picker current

The domain column showed a dash unless a hive had a dashboard but sorted
and filtered on the domain alone, so rows looked mis-sorted. It now shows
the domain as text when there is no dashboard to link. An empty hive
roster says so. The schedule form's agent names reload with the schedules
and the form says when that load failed.

frontend/packages/swarm-ui/test/agent-options.test.mjs

@@ -0,0 +1,27 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import {
+  agentNamesFrom,
+  agentOptionsFor,
+} from "../src/pages/schedules/agent-options.ts";
+
+test("agentNamesFrom: sorted names, empty before the roster loads", () => {
+  assert.deepEqual(agentNamesFrom([{ name: "b" }, { name: "a" }]), ["a", "b"]);
+  assert.deepEqual(agentNamesFrom(null), []);
+});
+
+test("agentOptionsFor: roster names alone for a new schedule", () => {
+  assert.deepEqual(agentOptionsFor(["b", "a"]), ["a", "b"]);
+});
+
+test("agentOptionsFor: keeps an agent that left the roster, once", () => {
+  assert.deepEqual(agentOptionsFor(["a", "b"], { agents: ["b", "gone"] }), [
+    "a",
+    "b",
+    "gone",
+  ]);
+});
+
+test("agentOptionsFor: an empty roster still offers the schedule's agents", () => {
+  assert.deepEqual(agentOptionsFor([], { agents: ["x"] }), ["x"]);
+});