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/child/reference/index.blade.php

@@ -0,0 +1,23 @@
+@extends('layouts.child')
+@section('content')
+<h1 class="text-2xl font-bold text-indigo-700 mb-2">💡 Erinnerung</h1>
+<p class="text-slate-500 mb-6">Hier kannst du nachschauen, was du gerade nicht mehr weißt.</p>
+@foreach($categories as $key => $label)
+@php $group = $articles->get($key, collect()) @endphp
+@if($group->count())
+<div class="mb-6">
+  <h2 class="font-bold text-slate-600 text-sm uppercase tracking-wide mb-3">{{ $label }}</h2>
+  <div class="grid gap-3">
+    @foreach($group as $a)
+    <a href="{{ route('reference.show', $a->slug) }}"
+       class="bg-white rounded-2xl border border-slate-200 shadow-sm px-5 py-4 flex items-center gap-4 hover:border-indigo-300 hover:shadow-md transition-all">
+      <span class="text-3xl">{{ $a->icon }}</span>
+      <span class="font-semibold text-slate-700 text-base">{{ $a->title }}</span>
+      <span class="ml-auto text-slate-300 text-lg">›</span>
+    </a>
+    @endforeach
+  </div>
+</div>
+@endif
+@endforeach
+@endsection