feat(frontend): show fediverse handle @user@domain on local user profiles

thoughts-frontend/app/users/[username]/page.tsx

@@ -105,6 +105,12 @@ export default async function ProfilePage({ params }: ProfilePageProps) {
   const isOwnProfile = me?.username === user.username;
   const isFollowing = user.isFollowedByViewer;
 
+  const apiDomain = process.env.NEXT_PUBLIC_API_URL
+    ? new URL(process.env.NEXT_PUBLIC_API_URL).hostname
+    : null;
+  const fediverseHandle =
+    user.local && apiDomain ? `@${user.username}@${apiDomain}` : null;
+
   const authorDetails = new Map<string, { avatarUrl?: string | null }>();
   authorDetails.set(user.username, { avatarUrl: user.avatarUrl });
 
@@ -182,6 +188,11 @@ export default async function ProfilePage({ params }: ProfilePageProps) {
                 >
                   @{user.username}
                 </p>
+                {fediverseHandle && (
+                  <p className="text-xs text-muted-foreground/70 mt-0.5 font-mono select-all">
+                    {fediverseHandle}
+                  </p>
+                )}
               </div>
 
               <p