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/server/src/routes/federation.ts

@@ -1965,7 +1965,7 @@ function processOwnershipTransferEvent(
 
   db.update(schema.dmChannels)
     .set({
-      ownerId: newOwnerLocal?.id ?? channel.ownerId,
+      ownerId: newOwnerLocal?.id ?? null,
       ownerHomeUserId: event.ownership.newOwner.homeUserId,
       ownerHomeInstance: event.ownership.newOwner.homeInstance,
     })
@@ -1975,7 +1975,7 @@ function processOwnershipTransferEvent(
   connectionManager.sendToDmMembers(channel.id, {
     type: 'dm_owner_updated',
     dmChannelId: channel.id,
-    newOwnerId: newOwnerLocal?.id ?? channel.ownerId!,
+    newOwnerId: newOwnerLocal?.id ?? event.ownership.newOwner.homeUserId,
   });
 
   const prevOwnerLocal = event.ownership.previousOwner
@@ -1991,7 +1991,7 @@ function processOwnershipTransferEvent(
     userId: prevOwnerId,
     content: JSON.stringify({
       event: 'owner_changed',
-      newOwnerId: newOwnerLocal?.id ?? channel.ownerId,
+      newOwnerId: newOwnerLocal?.id ?? event.ownership.newOwner.homeUserId,
       newOwnerDisplayName: newOwnerLocal?.displayName ?? newOwnerBaseName,
     }),
     type: 'system',
@@ -2006,7 +2006,7 @@ function processOwnershipTransferEvent(
       userId: prevOwnerId,
       content: JSON.stringify({
         event: 'owner_changed',
-        newOwnerId: newOwnerLocal?.id ?? channel.ownerId,
+        newOwnerId: newOwnerLocal?.id ?? event.ownership.newOwner.homeUserId,
         newOwnerDisplayName: newOwnerLocal?.displayName ?? newOwnerBaseName,
       }),
       type: 'system',