Add Erinnerung (reference) section with articles for Deutsch, Mathe, Englisch

- Migration, model, admin+child controllers, all views
- 17 seeded articles: Nomen, Verben, Adjektive, Zeitformen, Satzarten,
  Wortarten, Ganze Zahlen, Grundrechenarten, Einmaleins, Geometrie,
  Brüche, Zahlen/Farben/Wochentage/Sätze/Tiere auf Englisch
- CSS for .ref-content (h2/h3/p/ul/table + .beispiel/.tipp/.merke blocks)
- Child nav + admin sidebar links

resources/views/layouts/admin.blade.php

@@ -24,6 +24,7 @@
         <span>❓</span> Fragen
       </a>
       <a href="{{ route('admin.rewards.index') }}" class="flex items-center gap-3 px-3 py-2 rounded-lg {{ request()->routeIs('admin.rewards.*') ? 'bg-violet-600 text-white' : 'text-slate-300 hover:bg-slate-800' }}">
+      <a href="{{ route('admin.reference.index') }}" class="flex items-center gap-3 px-3 py-2 rounded-lg {{ request()->routeIs('admin.reference.*') ? 'bg-violet-600 text-white' : 'text-slate-300 hover:bg-slate-800' }}">💡 Erinnerung</a>
         <span>🎁</span> Belohnungen
       </a>
       <a href="{{ route('admin.redemptions.index') }}" class="flex items-center gap-3 px-3 py-2 rounded-lg {{ request()->routeIs('admin.redemptions.*') ? 'bg-violet-600 text-white' : 'text-slate-300 hover:bg-slate-800' }}">

resources/views/layouts/child.blade.php

@@ -14,6 +14,7 @@
     <div class="flex items-center gap-4">
       <a href="{{ route('learn.subjects') }}" class="text-sm font-medium text-slate-600 hover:text-indigo-600">Lernen</a>
       <a href="{{ route('rewards.index') }}" class="text-sm font-medium text-slate-600 hover:text-indigo-600">🪙 Belohnungen</a>
+      <a href="{{ route('reference.index') }}" class="text-sm font-medium text-slate-600 hover:text-indigo-600">💡 Erinnerung</a>
       <div class="flex items-center gap-1 bg-amber-100 text-amber-700 font-bold rounded-full px-3 py-1 text-sm">
         🪙 {{ auth()->user()->points }}
       </div>