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/hooks/useWebSocket.ts

@@ -8,6 +8,7 @@ import { useSettingsStore } from '../stores/settingsStore';
 import type { ServerEvent, ClientEvent, ActiveCallInfo } from '@backspace/shared';
 import { resolveAssetUrl, normalizeUserAssets, normalizeMessageAssets } from '../utils/assetUrls';
 import { broadcastVoiceStatus, broadcastDeafenViaLiveKit } from '../utils/voice';
+import { registerSelfId } from '../utils/identity';
 
 // ─── Connection state ─────────────────────────────────────────────────────────
 
@@ -88,6 +89,9 @@ function handleEvent(origin: string, event: ServerEvent): void {
 
   switch (event.type) {
     case 'ready':
+      // Register this user's ID for cross-instance self-identification
+      registerSelfId(event.user.id);
+
       if (isHome) {
         setUser(event.user);
         useSettingsStore.getState().setIsAdmin(event.user.isAdmin ?? false);