feat(messaging-ui): channels — browse, join, leave, create (contract + mock + UI)

Adds a third membership beyond dm/group: a discoverable, joinable room.

- contract: Membership += 'channel'; Conversation.topic; ChannelSummary +
  CreateChannelInput; optional adapter methods browseChannels/createChannel/
  joinChannel/leaveChannel (capability-degrading — absent hides the UI)
- MockAdapter implements them (seeds a joined public, a joinable public, and a
  private channel); listConversations now returns only threads I'm in
- useChannels hook (browse/create/join/leave + supported flag)
- UI: sidebar sections (Channels vs Direct messages), a ＋ that opens a
  ChannelBrowser (join + create), # / lock glyphs; themeable styles
- KernelClientAdapter passes channel membership + topic through
- 4 channel tests (mock browse/join/create + render browse→join); 52 total green

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

packages/iios-messaging-ui/src/adapters/kernel-client.ts

@@ -188,7 +188,9 @@ export class KernelClientAdapter implements MessagingAdapter {
 
   private toConversation(t: ThreadSummary): Conversation {
     const others = t.participants.filter((p) => p !== this.me);
-    const membership: Membership | null = t.membership === 'dm' || t.membership === 'group' ? t.membership : null;
+    const membership: Membership | null =
+      t.membership === 'dm' || t.membership === 'group' || t.membership === 'channel' ? t.membership : null;
+    const topic = (t.metadata as { topic?: string } | null)?.topic;
     return {
       threadId: t.threadId,
       title: t.subject?.trim() || others.join(', ') || 'Conversation',
@@ -196,6 +198,7 @@ export class KernelClientAdapter implements MessagingAdapter {
       membership,
       participants: [...t.participants],
       unread: t.unread,
+      ...(topic != null ? { topic } : {}),
       ...(t.lastMessage ? { lastMessage: t.lastMessage } : {}),
       ...(t.lastAt ? { lastAt: t.lastAt } : {}),
     };