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/Dashboard.jsx

@@ -56,7 +56,7 @@ const Dashboard = () => {
                     if (data.steamId) {
                         await fetchSteamCount();
                     }
-                } else if (profileResponse.status === 401) {
+                } else if (profileResponse.status === 401 || profileResponse.status === 403) {
                     localStorage.removeItem('tbp_jwt');
                     navigate('/login');
                 } else {
@@ -107,6 +107,17 @@ const Dashboard = () => {
                     <p className="text-lg text-slate-600 dark:text-slate-300">
                         {userData?.dashboardMessage}
                     </p>
+                    {userData?.steamId ? (
+                        <p className="mt-2 text-sm text-slate-500 dark:text-slate-400">
+                            <i className="mdi mdi-steam mr-1"></i>
+                            {t('SETTINGS.STEAM_ID')}: <span className="font-mono">{userData.steamId}</span>
+                        </p>
+                    ) : (
+                        <Link to="/settings" className="mt-2 inline-block text-sm text-blue-600 dark:text-blue-400 hover:underline">
+                            <i className="mdi mdi-steam mr-1"></i>
+                            {t('DASHBOARD.NO_STEAM_ID')}
+                        </Link>
+                    )}
                     {userData?.steamId && (
                         <div className="mt-4">
                             <SyncStatusBadge