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.

public/i18n/de.json

@@ -69,7 +69,8 @@
   "DASHBOARD": {
     "LOADING": "Lade Dashboard ...",
     "HELLO": "Hallo",
-    "STEAM_GAMES": "Steam Spiele"
+    "STEAM_GAMES": "Steam Spiele",
+    "NO_STEAM_ID": "Keine Steam-ID hinterlegt – jetzt in den Einstellungen eintragen"
   },
   "LIBRARY": {
     "TITLE": "Spielebibliothek",
@@ -130,5 +131,11 @@
     "GET_STARTED": "Jetzt loslegen",
     "LOGIN": "Anmelden",
     "LOGOUT": "Abmelden"
+  },
+  "NAV": {
+    "DASHBOARD": "Dashboard",
+    "LIBRARY": "Bibliothek",
+    "SETTINGS": "Einstellungen",
+    "TO_DASHBOARD": "Zum Dashboard"
   }
 }