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;

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);

packages/web/src/stores/authStore.ts

@@ -8,6 +8,7 @@ import { useVoiceStore } from './voiceStore';
 import { useInstanceStore } from './instanceStore';
 import { syncProfileUpdateToRemotes } from '../utils/profileSync';
 import { changePasswordOnRemotes, deleteAccountOnRemotes, type FederationOpResult } from '../utils/federationOps';
+import { clearSelfIds } from '../utils/identity';
 
 interface AuthState {
   token: string | null;
@@ -27,6 +28,7 @@ interface AuthState {
 
 /** Reset all user-scoped stores to prevent data leaking between sessions */
 function resetUserStores() {
+  clearSelfIds();
   useChatStore.getState().clearAllMessages();
   useSpaceStore.getState().populateFromReady('', [], [], []);
   useSocialStore.getState().reset();

packages/web/src/utils/identity.ts

@@ -11,6 +11,20 @@ export function parseFederatedUsername(username: string): { baseName: string; do
   return { baseName: username.slice(0, atIndex), domain: username.slice(atIndex + 1) };
 }
 
+// ─── Cross-instance self-ID registry ─────────────────────────────────────────
+// Tracks all Snowflake IDs that belong to the current user across connected
+// instances (home + remotes). Populated from WS `ready` events.
+
+const _knownSelfIds = new Set<string>();
+
+export function registerSelfId(id: string): void {
+  _knownSelfIds.add(id);
+}
+
+export function clearSelfIds(): void {
+  _knownSelfIds.clear();
+}
+
 /**
  * Stateless check: is `user` a replicated alias of `homeUser`?
  * Uses the immutable (username, homeInstance) composite key —
@@ -23,12 +37,15 @@ export function isSelf(
   if (!homeUser) return false;
   // Same instance, same ID — trivial case
   if (user.id === homeUser.id) return true;
+  // Cross-instance: check all known user IDs from connected instances
+  if (_knownSelfIds.has(user.id)) return true;
   // Replicated user: homeInstance matches our origin
   if (!user.homeInstance) return false;
   if (user.homeInstance !== window.location.host) return false;
   // Username: "youruser" or "youruser@nova.ddns.net" → base must match
   const { baseName } = parseFederatedUsername(user.username);
-  return baseName === homeUser.username;
+  const { baseName: homeBase } = parseFederatedUsername(homeUser.username);
+  return baseName === homeBase;
 }
 
 /**