feat: update frontend to work with v2 backend — camelCase, new endpoints, nested author

thoughts-frontend/app/thoughts/[thoughtId]/page.tsx

@@ -15,7 +15,7 @@ interface ThoughtPageProps {
 }
 
 function collectAuthors(thread: ThoughtThreadType): string[] {
-  const authors = new Set<string>([thread.authorUsername]);
+  const authors = new Set<string>([thread.author.username]);
   for (const reply of thread.replies) {
     collectAuthors(reply).forEach((author) => authors.add(author));
   }