swarm-ui: a broadcast page that sends one message to several agents

Pick agents from a grid of their icons (running or all), type the text, send.
The page posts the names and the text to the controller and shows what
happened to each agent, with a retry that selects just the ones that failed.
The selection rules and the controller's answer are read by plain functions
with tests.

frontend/packages/swarm-ui/src/App.tsx

@@ -6,6 +6,7 @@ import { AgentsPage } from "./pages/agents/AgentsPage.js";
 import { AgentTerminalPage } from "./pages/agents/AgentTerminalPage.js";
 import { AlertsPage } from "./pages/AlertsPage.js";
 import { BuildsPage } from "./pages/builds/BuildsPage.js";
+import { BroadcastPage } from "./pages/broadcast/BroadcastPage.js";
 import { ComponentsPage } from "./pages/ComponentsPage.js";
 import { JobsPage } from "./pages/JobsPage.js";
 import { HivesPage } from "./pages/HivesPage.js";
@@ -39,6 +40,7 @@ export function App() {
         <Route path="/jobs" component={JobsPage} />
         <Route path="/schedules" component={SchedulesPage} />
         <Route path="/builds" component={BuildsPage} />
+        <Route path="/broadcast" component={BroadcastPage} />
         <Route path="/issues" component={IssueReportPage} />
         <Route path="/review" component={ReviewPage} />
         <Route path="/components" component={ComponentsPage} />