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/show.blade.php

@@ -0,0 +1,28 @@
+@extends('layouts.child')
+@section('content')
+<div class="mb-5">
+  <a href="{{ route('reference.index') }}" class="text-sm text-slate-400 hover:text-slate-600">← Erinnerung</a>
+</div>
+<div class="bg-white rounded-2xl shadow-sm border border-slate-200 p-6 mb-6">
+  <div class="flex items-center gap-3 mb-5 pb-4 border-b border-slate-100">
+    <span class="text-4xl">{{ $reference->icon }}</span>
+    <h1 class="text-2xl font-black text-slate-800">{{ $reference->title }}</h1>
+  </div>
+  <div class="ref-content">
+    {!! $reference->content !!}
+  </div>
+</div>
+@if($others->count())
+<h2 class="font-bold text-slate-600 text-sm mb-3">Weitere Artikel</h2>
+<div class="grid gap-2">
+  @foreach($others as $a)
+  <a href="{{ route('reference.show',$a->slug) }}"
+     class="bg-white rounded-xl border border-slate-200 px-4 py-3 flex items-center gap-3 hover:border-indigo-300 transition-all text-sm">
+    <span class="text-xl">{{ $a->icon }}</span>
+    <span class="font-medium text-slate-700">{{ $a->title }}</span>
+    <span class="ml-auto text-slate-300">›</span>
+  </a>
+  @endforeach
+</div>
+@endif
+@endsection