activities: use sessionStorage for selected tab

The useLocalStorage hook synchronizes state between windows. In the
case of the activities tabs, we don't want this state synchronized,
otherwise changing a tab in one window would change the tab in all
open windows.

Instead, we can use sessionStorage so that the state persists when
refreshing or duplicating a browser tab. Local storage is still used
as the default value so that any new window that is opened will use
the last selected state.

Issue: https://github.com/pybricks/support/issues/807

src/app/App.test.tsx

@@ -18,6 +18,7 @@ afterEach(() => {
     cleanup();
     jest.resetAllMocks();
     localStorage.clear();
+    sessionStorage.clear();
 });
 
 it.each([false, true])('should render', (darkMode) => {