Revert "Make /activity public so signed-out UI renders"

This reverts commit 5419f01d54.

frontend/src/proxy.ts

@@ -9,9 +9,7 @@ const isClerkEnabled = () =>
   );
 
 // Public routes must include Clerk sign-in paths to avoid redirect loops.
-// Also keep top-level UI routes like /activity public so the app can render a signed-out state
-// (the page itself shows a SignIn button; API routes remain protected elsewhere).
-const isPublicRoute = createRouteMatcher(["/sign-in(.*)", "/activity(.*)"]);
+const isPublicRoute = createRouteMatcher(["/sign-in(.*)"]);
 
 export default isClerkEnabled()
   ? clerkMiddleware(async (auth, req) => {