feat(nav): add header navigation and show Steam ID on dashboard

Header links to Dashboard, Library and Settings. The logo, the login page and
the landing page lead logged-in users to the dashboard. Expired tokens are
removed, and a 403 on the dashboard logs out. Remove the dev dummy token.

src/pages/LandingPage.jsx

@@ -1,5 +1,6 @@
 import {useTranslation} from 'react-i18next';
 import {Link} from 'react-router-dom';
+import {getValidToken} from '../utils/auth.js';
 import StatCard from '../components/StatCard.jsx';
 
 import usePublicStats from '../hooks/usePublicStats.js';
@@ -25,9 +26,15 @@ export default function LandingPage() {
                 <p className="text-lg text-slate-600 dark:text-slate-400 max-w-2xl mx-auto mb-8">
                     {t('MAIN.DESCRIPTION')}
                 </p>
-                <Link to="/login" className="bg-blue-600 hover:bg-blue-500 text-white px-8 py-3 rounded-xl font-bold text-lg transition-all shadow-lg shadow-blue-500/30">
-                    {t('MAIN.GET_STARTED')}
-                </Link>
+                {getValidToken() ? (
+                    <Link to="/dashboard" className="bg-blue-600 hover:bg-blue-500 text-white px-8 py-3 rounded-xl font-bold text-lg transition-all shadow-lg shadow-blue-500/30">
+                        {t('NAV.TO_DASHBOARD')}
+                    </Link>
+                ) : (
+                    <Link to="/login" className="bg-blue-600 hover:bg-blue-500 text-white px-8 py-3 rounded-xl font-bold text-lg transition-all shadow-lg shadow-blue-500/30">
+                        {t('MAIN.GET_STARTED')}
+                    </Link>
+                )}
             </section>
 
             <section className="w-full max-w-5xl mb-16">