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/components/conversation-list.tsx

@@ -33,7 +33,7 @@ export function ConversationList({
             onClick={() => onSelect(c.threadId)}
           >
             <span className="miu-avatar" aria-hidden="true">
-              {initials(c.title)}
+              {c.membership === 'channel' ? '#' : initials(c.title)}
             </span>
             <span className="miu-convrow-main">
               <span className="miu-convrow-title">{c.title}</span>