test: add e2e test for system time checkbox

cypress/e2e/clock.cy.ts

@@ -209,4 +209,30 @@ describe("Berlin Clock", () => {
       assertFullClock("11:37:01", "ORROOROOOYYRYYRYOOOOYYOO");
     });
   });
+
+  describe("Use System Time", () => {
+    it("should show system time when checkbox is checked", () => {
+      // Start by entering a fixed time
+      cy.get('[data-testid="time-input"]').clear().type("12:00:00").type("{enter}");
+      cy.get('[data-testid="digital-clock"]').should("contain", "12:00:00");
+
+      // Check the "Use system time" checkbox
+      cy.get('[data-testid="use-system-time"]').check();
+
+      // Now the digital clock should reflect the real system time
+      cy.get('[data-testid="digital-clock"]').invoke("text").then((displayedTime) => {
+        // displayedTime is "HH:mm:ss"
+        const parts = displayedTime.split(":");
+        const displayedSeconds =
+          parseInt(parts[0]) * 3600 +
+          parseInt(parts[1]) * 60 +
+          parseInt(parts[2]);
+        const now = new Date();
+        const realSeconds =
+          now.getHours() * 3600 + now.getMinutes() * 60 + now.getSeconds();
+        // Allow a 5-second tolerance
+        expect(Math.abs(displayedSeconds - realSeconds)).to.be.lessThan(5);
+      });
+    });
+  });
 });