fix: three code review issues — leaveGroup duplicate, as any casts, federation ownership fallback

- Remove redundant `leaveGroup` API method from client.ts (duplicated `leave`); update MessageList.tsx WelcomeHeader to call `api.dm.leave` directly
- Add optional `type` field to shared `Message` interface so `MessageWithUser` carries it; remove `(msg as any).type` casts in `isSameGroup` and the render branch in MessageList.tsx
- Fix `processOwnershipTransferEvent` in federation.ts: replace `channel.ownerId` fallbacks (pre-update, old owner) with `event.ownership.newOwner.homeUserId` in the db update, dm_owner_updated broadcast, and both system message content payloads

packages/shared/src/types.ts

@@ -193,6 +193,7 @@ export interface Message {
   userId: string;
   replyToId: string | null;
   content: string | null;
+  type?: 'user' | 'system';
   editedAt: number | null;
   createdAt: number;
 }