fix: resolve duplicate voice user in federated channel sidebar

Use origin-aware user ID resolution for optimistic voice add/remove
instead of always using the home instance ID. Adds getMyUserIdForOrigin
resolver (same pattern as getApiForOrigin) so joinVoiceChannel and
leaveVoice use the correct federated user ID. Also fixes WS reconnect
voice re-registration, invite URL for remote servers, and chatStore
early-load guard for unmapped channels.

packages/web/src/stores/instanceStore.ts

@@ -2,7 +2,7 @@ import { create } from 'zustand';
 import type { User, InstanceInfoResponse, ReplicatedInstance, AuthResponse } from '@backspace/shared';
 import { BackspaceApiClient, createApiClient, api } from '../api/client';
 import { useAuthStore } from './authStore';
-import { setApiForOriginResolver, useServerStore } from './serverStore';
+import { setApiForOriginResolver, setUserIdForOriginResolver, useServerStore } from './serverStore';
 import { connectInstance, disconnectInstance, disconnectAllRemote } from '../hooks/useWebSocket';
 
 // ─── Types ───────────────────────────────────────────────────────────────────
@@ -505,3 +505,12 @@ setApiForOriginResolver((origin: string): BackspaceApiClient => {
   if (!instance) return api;
   return instance.api;
 });
+
+// ─── User ID resolution (federation) ──────────────────────────────────────────
+// Maps an origin to the local user's ID on that remote instance.
+// Used by voice join/leave to optimistically add/remove the correct user ID.
+
+setUserIdForOriginResolver((origin: string): string | undefined => {
+  const instance = useInstanceStore.getState().instances.find(i => i.origin === origin);
+  return instance?.user.id;
+});