fix(frontend): prevent RangeError in TimeAdjuster and add validation to SleepButton

frontend/tests/unit/TimeAdjuster.test.tsx

@@ -29,4 +29,18 @@ describe('TimeAdjuster', () => {
     fireEvent.click(resetButton);
     expect(handleChange).toHaveBeenCalled();
   });
+
+  it('handles invalid date input gracefully (returns empty string)', async () => {
+    // Since we changed the implementation, it should no longer throw.
+    // We can't easily test the useEffect error anymore because it doesn't throw,
+    // but we can check if onTimeChange is called with "" for invalid input.
+    const handleChange = vi.fn();
+    const { container } = render(<TimeAdjuster onTimeChange={handleChange} />);
+    const input = container.querySelector('input');
+
+    fireEvent.change(input!, { target: { value: '2026-06-0' } });
+    // We might need to wait for the effect
+    await new Promise((r) => setTimeout(r, 0));
+    expect(handleChange).toHaveBeenCalledWith("");
+  });
 });