test(e2e): add Clerk OTP login helper (cy.origin)

frontend/cypress/e2e/clerk_login.cy.ts

@@ -0,0 +1,19 @@
+describe("Clerk login (OTP)", () => {
+  it("can sign in via Clerk modal", () => {
+    // Skip unless explicitly configured.
+    const clerkOrigin = Cypress.env("CLERK_ORIGIN");
+    const email = Cypress.env("CLERK_TEST_EMAIL");
+    const otp = Cypress.env("CLERK_TEST_OTP");
+
+    if (!clerkOrigin || !email || !otp) {
+      cy.log("Skipping: missing CYPRESS_CLERK_ORIGIN / CYPRESS_CLERK_TEST_EMAIL / CYPRESS_CLERK_TEST_OTP");
+      return;
+    }
+
+    cy.visit("/activity");
+    cy.loginWithClerkOtp();
+
+    // After login, the SignedIn UI should render.
+    cy.contains(/live feed/i, { timeout: 20_000 }).should("be.visible");
+  });
+});