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

frontend/tests/unit/SleepButton.test.tsx

@@ -70,4 +70,20 @@ describe('SleepButton', () => {
 
     await waitFor(() => expect(endSessionMock).toHaveBeenCalled());
   });
+
+  it('shows alert when time is invalid', async () => {
+    const alertSpy = vi.spyOn(window, 'alert').mockImplementation(() => {});
+    (useSleepSession as any).mockReturnValue({
+      activeSession: null,
+      isCreating: false,
+      isEnding: false,
+    });
+
+    renderWithProviders(<SleepButton time="" />);
+    const button = screen.getByRole('button', { name: /Start Sleep/i });
+    fireEvent.click(button);
+
+    expect(alertSpy).toHaveBeenCalledWith('Please select a valid time.');
+    alertSpy.mockRestore();
+  });
 });