swarm-ui: the broadcast composer is its own folded card above the messages card

Replaces the broadcast button in the messages card's header: a Disclosure
titled broadcast, folded by default, over the messages card. The composer is
built on the first open and kept, so it polls the agent list only once used.
The broadcast page wording in broadcast-types.ts and its test now says
composer.

frontend/packages/swarm-ui/test/broadcast-types.test.mjs

@@ -1,4 +1,4 @@
-// The broadcast page's selection and answer rules. Run by `npm test`, which
+// The broadcast composer's selection and answer rules. Run by `npm test`, which
 // Node runs with its built-in test runner and type stripping, so the `.ts`
 // module is imported as it is.
 import assert from "node:assert/strict";