E2E: add /sign-in redirect; use it in Clerk Cypress login helper

frontend/src/app/sign-in/page.tsx

@@ -0,0 +1,14 @@
+import { redirect } from "next/navigation";
+import { auth } from "@clerk/nextjs/server";
+
+export default function SignInPage() {
+  const { userId, redirectToSignIn } = auth();
+
+  if (userId) {
+    redirect("/activity");
+  }
+
+  // Top-level redirect to Clerk hosted sign-in.
+  // Cypress E2E cannot reliably drive Clerk modal/iframe login.
+  return redirectToSignIn({ returnBackUrl: "/activity" });
+}