feat: add instance store and Connected Instances settings UI

Introduce instanceStore (Zustand) with full federation lifecycle:
probe remote instances, register with username collision fallback,
login to existing accounts, sync instance list across all connected
instances, auto-reconnect from cached tokens on login/page load, and
cleanup on logout. Add ConnectedInstances component to User Settings
with home instance card, remote instance management, and inline
add-instance flow (URL probe → register/login → connected).

packages/web/src/components/modals/UserSettings.tsx

@@ -4,6 +4,7 @@ import { useUIStore } from '../../stores/uiStore';
 import { useAuthStore } from '../../stores/authStore';
 import { useVoiceStore } from '../../stores/voiceStore';
 import { Avatar } from '../ui/Avatar';
+import { ConnectedInstances } from './ConnectedInstances';
 
 export function UserSettingsModal() {
   const activeModal = useUIStore((s) => s.activeModal);
@@ -168,6 +169,9 @@ export function UserSettingsModal() {
           </div>
         </div>
 
+        {/* Connected Instances */}
+        <ConnectedInstances />
+
         <div className="flex items-center justify-between pt-2">
           <button
             onClick={handleLogout}