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/web/src/api/client.ts

@@ -145,7 +145,6 @@ export class BackspaceApiClient {
     deleteMessage: (id: string) => Promise<{ success: boolean }>;
     addMember: (dmChannelId: string, data: { userId: string }) => Promise<DmChannel>;
     leave: (dmChannelId: string) => Promise<{ success: boolean }>;
-    leaveGroup: (dmChannelId: string) => Promise<{ success: boolean }>;
   };
 
   readonly social: {
@@ -506,8 +505,6 @@ export class BackspaceApiClient {
         request<DmChannel>('POST', `/dm/${dmChannelId}/members`, data),
       leave: (dmChannelId: string) =>
         request<{ success: boolean }>('DELETE', `/dm/${dmChannelId}/members`),
-      leaveGroup: (dmChannelId: string) =>
-        request<{ success: boolean }>('DELETE', `/dm/${dmChannelId}/members`),
     };
 
     this.social = {