feat: public invite landing page with federation redirect

Make /join/:code a public route with a standalone JoinPage that shows a
space preview and handles authenticated, unauthenticated, and cross-instance
users. Adds GET /api/spaces/invite/:code/preview (no auth) endpoint,
?redirect= param support on login/register, and cleans up dead invite
handling from AppLayout and JoinSpace modal.

packages/web/src/components/layout/AppLayout.tsx

@@ -34,7 +34,7 @@ import { useVoiceStore } from '../../stores/voiceStore';
 import { AudioManager } from '../../audio/AudioManager';
 
 export function AppLayout() {
-  const { spaceId, channelId, inviteCode } = useParams<{ spaceId?: string; channelId?: string; inviteCode?: string }>();
+  const { spaceId, channelId } = useParams<{ spaceId?: string; channelId?: string }>();
   const navigate = useNavigate();
   
   // Global interaction handler to resume AudioContext
@@ -91,7 +91,6 @@ export function AppLayout() {
   const loadMessages = useChatStore((s) => s.loadMessages);
   const setIsMobile = useUIStore((s) => s.setIsMobile);
   const setShowDms = useUIStore((s) => s.setShowDms);
-  const openModal = useUIStore((s) => s.openModal);
 
   const sidebarOpen = useUIStore((s) => s.sidebarOpen);
   const isMobile = useUIStore((s) => s.isMobile);
@@ -200,12 +199,6 @@ export function AppLayout() {
     }
   }, [spaceId, setCurrentSpace, loadSpaceDetail, setShowDms]);
 
-  useEffect(() => {
-    if (inviteCode) {
-      openModal('joinSpace');
-    }
-  }, [inviteCode, openModal]);
-
   useEffect(() => {
     if (channelId) {
       setCurrentChannel(channelId);