fix: redirect back to activity.bincio.org after login via bincio.org

Pass ?next=<current URL> when bouncing to bincio.org/login/ so the user
lands back on the activity page they came from instead of bincio.org/.

site/src/layouts/Base.astro

@@ -104,7 +104,8 @@ try {
       fetch('/api/me', { credentials: 'include' })
         .then(r => {
           if (r.status === 401 || r.status === 404) {
-            window.location.replace(authUrl ? authUrl + '/login/' : '/login/');
+            const next = authUrl ? '?next=' + encodeURIComponent(window.location.href) : '';
+            window.location.replace(authUrl ? authUrl + '/login/' + next : '/login/');
           } else {
             document.body.removeAttribute('data-auth-pending');
           }