feat(messaging-ui): rendered Messenger components — the drop-in UI

Turns the SDK from hooks-only into a real UI SDK: <Messenger> (conversation
list + open thread + composer), plus <ConversationList> and <Thread>, all
driven by the existing useConversations/useMessages hooks over the injected
adapter — zero transport imports (boundary intact).

- themeable via --miu-* CSS variables; default theme ships as ./styles.css
- optimistic send, typing indicator, seen ticks, reactions display, unread badges
- render smoke tests (jsdom + MockAdapter): mounts, auto-selects first thread,
  sends a message, switches threads (3 tests) — 48 total green
- tsup: css bundled to dist/styles.css; dts scoped to TS entries

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/index.ts

@@ -8,6 +8,11 @@ export { useConversations } from './hooks/use-conversations';
 export { useMessages } from './hooks/use-messages';
 export { isOwnMessage } from './types';
 
+// Rendered UI. Pair with the './styles.css' export (or override the --miu-* tokens).
+export { Messenger } from './components/messenger';
+export { ConversationList } from './components/conversation-list';
+export { Thread } from './components/thread';
+
 export type { MessagingAdapter } from './adapter';
 export type { ConversationsState } from './hooks/use-conversations';
 export type { MessagesState, UiMessage } from './hooks/use-messages';