Enable 'Explore' sidebar link for logged-in users

This change ensures the 'Explore' link is visible in the sidebar when the user is logged in, and hidden when logged out, consistent with other authenticated routes like 'For You' and 'Subscriptions'.

- Updated `js/app.js` to handle `exploreLink` visibility in `applyAuthenticatedUiState` and `applyLoggedOutUiState`.
- Initialized `exploreLink` property in `js/ui/applicationBootstrap.js`.
- Updated `tests/app/hydrate-sidebar-navigation.test.mjs` to verify the behavior.

js/ui/applicationBootstrap.js

@@ -657,6 +657,7 @@ export default class ApplicationBootstrap {
 
     app.subscriptionsLink = null;
     app.forYouLink = null;
+    app.exploreLink = null;
 
     app.notificationPortal = doc?.getElementById("notificationPortal") || null;
     app.errorContainer = doc?.getElementById("errorContainer") || null;