test(e2e): add Cypress scaffold + CI job

frontend/cypress/e2e/activity_smoke.cy.ts

@@ -0,0 +1,7 @@
+describe("/activity page", () => {
+  it("loads without crashing", () => {
+    cy.visit("/activity");
+    // In secretless/unsigned state, UI should render the signed-out prompt.
+    cy.contains(/sign in to view the feed/i).should("be.visible");
+  });
+});