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

frontend/cypress/support/commands.ts

@@ -34,8 +34,9 @@ Cypress.Commands.add("loginWithClerkOtp", () => {
 
   const opts: ClerkOtpLoginOptions = { clerkOrigin, email, otp };
 
-  // Trigger the modal from the app first.
-  cy.get('[data-testid="activity-signin"]').click({ force: true });
+  // Navigate to a dedicated sign-in route that performs a top-level redirect
+  // to Clerk hosted sign-in (avoids modal/iframe limitations in Cypress).
+  cy.visit("/sign-in");
 
   // The Clerk UI is typically hosted on a different origin (clerk.accounts.dev / clerk.com).
   // Use cy.origin to drive the UI in Chrome.

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" });
+}