fix: federation DM identity resolution — cross-instance isSelf() failure

Add a cross-instance self-ID registry to identity.ts so isSelf() can
recognize the current user's Snowflake IDs from all connected instances.
Previously, federated DMs showed the user themselves as the other party
because remote-instance IDs didn't match the home user ID.

- Register user IDs from every WS ready event (home + remote)
- Clear the registry on session reset (login/logout/register/delete)
- Fix isSelf() username comparison to parse both sides as federated
- Replace naive ID check in MessageList WelcomeHeader with isSelf()

packages/web/src/components/chat/MessageList.tsx

@@ -7,6 +7,7 @@ import { useSocialStore } from '../../stores/socialStore';
 import { Avatar } from '../ui/Avatar';
 import { LoadingSpinner } from '../ui/LoadingSpinner';
 import { hasPermissionBit, PermissionBits } from '../../utils/permissions';
+import { isSelf } from '../../utils/identity';
 import type { MessageWithUser } from '@backspace/shared';
 
 const EMPTY_MESSAGES: MessageWithUser[] = [];
@@ -247,7 +248,7 @@ function WelcomeHeader({ channelId }: { channelId: string }) {
 
   if (isDm) {
     const dm = dmChannels.find(d => d.id === channelId);
-    const otherUser = dm?.members.find(m => m.id !== authUser?.id);
+    const otherUser = dm?.members.find(m => !isSelf(m, authUser));
     const displayName = otherUser?.displayName ?? otherUser?.username ?? 'Unknown';
     const username = otherUser?.username ?? 'unknown';
     const isFriend = otherUser ? friends.some(f => f.id === otherUser.id) : false;