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/admin/reference/index.blade.php

@@ -0,0 +1,33 @@
+@extends('layouts.admin')
+@section('title','Erinnerung')
+@section('content')
+<div class="flex justify-between items-center mb-6">
+  <h2 class="text-lg font-semibold text-slate-700">Erinnerungs-Artikel</h2>
+  <a href="{{ route('admin.reference.create') }}" class="bg-violet-600 hover:bg-violet-700 text-white px-4 py-2 rounded-lg text-sm font-medium">+ Neuer Artikel</a>
+</div>
+@foreach($categories as $key => $label)
+@php $group = $articles->get($key, collect()) @endphp
+<div class="mb-6">
+  <h3 class="font-semibold text-slate-600 text-sm mb-2">{{ $label }}</h3>
+  <div class="bg-white rounded-xl shadow-sm border border-slate-200 overflow-hidden">
+    @forelse($group as $a)
+    <div class="flex items-center justify-between px-4 py-3 {{ !$loop->last ? 'border-b border-slate-100' : '' }} hover:bg-slate-50">
+      <div class="flex items-center gap-3">
+        <span class="text-xl">{{ $a->icon }}</span>
+        <span class="font-medium text-slate-700">{{ $a->title }}</span>
+      </div>
+      <div class="flex gap-3 text-sm">
+        <a href="{{ route('admin.reference.edit',$a) }}" class="text-violet-600 hover:underline">Bearbeiten</a>
+        <form method="POST" action="{{ route('admin.reference.destroy',$a) }}" class="inline" onsubmit="return confirm('Löschen?')">
+          @csrf @method('DELETE')
+          <button class="text-red-500 hover:underline">Löschen</button>
+        </form>
+      </div>
+    </div>
+    @empty
+    <div class="px-4 py-4 text-slate-400 text-sm">Noch keine Artikel.</div>
+    @endforelse
+  </div>
+</div>
+@endforeach
+@endsection