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/shell/Shell.tsx

@@ -54,6 +54,7 @@ const NAV_ITEMS: { href: string; label: string; accent: string }[] = [
   { href: "/alerts", label: "alerts", accent: "var(--red)" },
   { href: "/jobs", label: "jobs", accent: "var(--pink)" },
   { href: "/schedules", label: "schedules", accent: "var(--cyan)" },
+  { href: "/broadcast", label: "broadcast", accent: "var(--blue)" },
   { href: "/issues", label: "issues", accent: "var(--yellow)" },
   { href: "/review", label: "review", accent: "var(--amber)" },
   { href: "/builds", label: "builds", accent: "var(--green)" },