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/components/messenger.test.tsx

@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import { MessagingProvider } from '../provider';
+import { Messenger } from './messenger';
+import { MockAdapter } from '../adapters/mock';
+
+function mount() {
+  return render(
+    <MessagingProvider adapter={new MockAdapter()}>
+      <Messenger />
+    </MessagingProvider>,
+  );
+}
+
+describe('<Messenger /> (rendered UI over an adapter)', () => {
+  it('renders the conversation list and auto-selects the first thread', async () => {
+    mount();
+    // Seeded group conversation title from the mock.
+    expect(await screen.findByText('Storm response — East side')).toBeTruthy();
+    // Auto-selected thread shows its seeded message.
+    expect(await screen.findByText('Crew is rolling out at 7.')).toBeTruthy();
+  });
+
+  it('sends a message through the adapter and shows it in the thread', async () => {
+    mount();
+    await screen.findByText('Crew is rolling out at 7.');
+
+    const input = screen.getByLabelText('Message') as HTMLInputElement;
+    fireEvent.change(input, { target: { value: 'on our way' } });
+    fireEvent.click(screen.getByText('Send'));
+
+    await waitFor(() => expect(screen.getByText('on our way')).toBeTruthy());
+    // Composer cleared after send.
+    expect(input.value).toBe('');
+  });
+
+  it('switches threads when another conversation is clicked', async () => {
+    mount();
+    // Click the DM (its title is the other participant's name from the mock directory).
+    fireEvent.click(await screen.findByText('Sofia Ramirez'));
+    expect(await screen.findByText('Can you review the Henderson estimate?')).toBeTruthy();
+  });
+});