Add did property to next-auth Session type

The spaces route accesses session.user.did for space ownership.
Adding the optional property to the type augmentation fixes the
TypeScript compilation error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types/next-auth.d.ts

@@ -4,6 +4,7 @@ declare module "next-auth" {
   interface Session {
     user: {
       id: string;
+      did?: string | null;
     } & DefaultSession["user"];
   }
 }