swarm-ui: broadcast is a composer on the messages page, not a tab of its own

The messages page gets a broadcast button that opens the agent picker and
message box above the stream, so a sent message shows up in the list below
it. The broadcast route and nav tab go, with no redirect: the page is now the
composer, BroadcastComposer, in the messages page.

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -54,7 +54,6 @@ 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: "/messages", label: "messages", accent: "var(--purple)" },
   { href: "/notifications", label: "notifications", accent: "var(--yellow)" },
   { href: "/issues", label: "issues", accent: "var(--yellow)" },