fix(activity): avoid Clerk hydration mismatch during initial render

frontend/src/app/activity/page.tsx

@@ -133,6 +133,15 @@ const FeedCard = memo(function FeedCard({
 FeedCard.displayName = "FeedCard";
 
 export default function ActivityPage() {
+  // Clerk auth state can differ between the server render and client hydration.
+  // When that happens, <SignedIn>/<SignedOut> can cause a React hydration mismatch
+  // that fails Cypress (and is noisy in the console). Gate the initial render until
+  // after mount so the first client render matches the server markup.
+  const [isMounted, setIsMounted] = useState(false);
+  useEffect(() => {
+    setIsMounted(true);
+  }, []);
+
   const { isSignedIn } = useAuth();
   const isPageActive = usePageActive();
 
@@ -294,6 +303,8 @@ export default function ActivityPage() {
 
   return (
     <DashboardShell>
+      {isMounted ? (
+        <>
           <SignedOut>
             <SignedOutPanel
               message="Sign in to view the feed."
@@ -334,6 +345,8 @@ export default function ActivityPage() {
               </div>
             </main>
           </SignedIn>
+        </>
+      ) : null}
     </DashboardShell>
   );
 }