feat: multi-WebSocket connection manager for instance federation

Refactor the WebSocket layer from a singleton connection to a connection
map supporting N concurrent instances. Each connection has its own
heartbeat worker, reconnect state, and token. Stores are now
instance-aware: serverStore merges servers by origin, chatStore routes
API calls through the correct client, and instanceStore triggers WS
connect/disconnect on add/remove. DM, social, and voice events remain
home-only.

packages/web/src/stores/authStore.ts

@@ -57,7 +57,7 @@ export const useAuthStore = create<AuthState>((set, get) => ({
     localStorage.removeItem('backspace_token');
     // Clear all user-scoped state to prevent data leaking between sessions
     useChatStore.getState().clearAllMessages();
-    useServerStore.getState().populateFromReady([], [], []);
+    useServerStore.getState().populateFromReady('', [], [], []);
     useSocialStore.getState().reset();
     useVoiceStore.getState().clearAllVoiceUsers();
     useInstanceStore.getState().reset();