fix: JoinPage graceful already-member handling + "I use another instance" for auth'd users

- Add spaceId to InvitePreview so the client can navigate to the space
- Detect "already a member" error and show green success card with auto-redirect
- Add "I use another instance" link for authenticated users alongside "Not you? Log in"

packages/shared/src/types.ts

@@ -53,6 +53,7 @@ export interface Space {
 }
 
 export interface InvitePreview {
+  spaceId: string;
   spaceName: string;
   description: string | null;
   icon: string | null;