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

database/migrations/2026_05_05_210000_create_reference_articles_table.php

@@ -0,0 +1,19 @@
+<?php
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+return new class extends Migration {
+    public function up(): void {
+        Schema::create('reference_articles', function (Blueprint $table) {
+            $table->id();
+            $table->string('title');
+            $table->string('slug')->unique();
+            $table->enum('category', ['deutsch','mathe','englisch']);
+            $table->string('icon')->default('📖');
+            $table->text('content');
+            $table->unsignedSmallInteger('sort_order')->default(0);
+            $table->timestamps();
+        });
+    }
+    public function down(): void { Schema::dropIfExists('reference_articles'); }
+};