feat: system messages, group DM welcome header, and dm_owner_updated event

- Render system messages (member_added, member_removed, owner_changed) inline
  in MessageList with icon + human-readable text; system messages never group
  with adjacent user messages
- Rewrite WelcomeHeader to branch on ownerId: group DMs show overlapping avatars,
  group name, creator attribution, federated privacy note, and a Leave Group button
- Add dm_owner_updated ServerEvent; broadcast from dm.ts leave handler and
  federation processOwnershipTransferEvent so all clients update ownerId in real-time
- Add updateDmOwner action to spaceStore and handle dm_owner_updated in useWebSocket
- Add leaveGroup alias to API client dm namespace

packages/web/src/stores/spaceStore.ts

@@ -53,6 +53,7 @@ interface SpaceState {
   removeDmChannel: (id: string) => void;
   addDmMember: (dmChannelId: string, user: User) => void;
   removeDmMember: (dmChannelId: string, userId: string) => void;
+  updateDmOwner: (dmChannelId: string, newOwnerId: string) => void;
   closeDm: (id: string) => Promise<void>;
   leaveDm: (id: string) => Promise<void>;
   loadSpaces: () => Promise<void>;
@@ -201,6 +202,12 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
     ),
   })),
 
+  updateDmOwner: (dmChannelId, newOwnerId) => set((state) => ({
+    dmChannels: state.dmChannels.map(dm =>
+      dm.id === dmChannelId ? { ...dm, ownerId: newOwnerId } : dm
+    ),
+  })),
+
   closeDm: async (id) => {
     const origin = get().channelOriginMap.get(id) || '';
     const targetApi = getApiForOrigin(origin);