Add grade/Klasse system: assign class levels to users, questions, and quizzes

- users.grade: set per child in admin (Klasse 1–10)
- quizzes.grade, questions.grade: optional target class (null = all)
- Children only see content matching their grade or without grade set
- Admin views show grade badge in user list, quiz list, questions list
- Quiz create/edit and user create/edit have Klasse dropdown

database/migrations/2026_05_06_000000_add_grade_to_users_quizzes_questions.php

@@ -0,0 +1,16 @@
+<?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::table('users',     fn(Blueprint $t) => $t->unsignedTinyInteger('grade')->nullable()->after('role'));
+        Schema::table('quizzes',   fn(Blueprint $t) => $t->unsignedTinyInteger('grade')->nullable()->after('active'));
+        Schema::table('questions', fn(Blueprint $t) => $t->unsignedTinyInteger('grade')->nullable()->after('active'));
+    }
+    public function down(): void {
+        Schema::table('users',     fn(Blueprint $t) => $t->dropColumn('grade'));
+        Schema::table('quizzes',   fn(Blueprint $t) => $t->dropColumn('grade'));
+        Schema::table('questions', fn(Blueprint $t) => $t->dropColumn('grade'));
+    }
+};