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/shared/src/types.ts

@@ -52,6 +52,15 @@ export interface Space {
   createdAt: number;
 }
 
+export interface InvitePreview {
+  spaceName: string;
+  description: string | null;
+  icon: string | null;
+  avatarColor: AvatarColor | null;
+  memberCount: number;
+  instanceName: string;
+}
+
 export interface ExploreSpace {
   id: string;
   name: string;