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

resources/views/admin/quizzes/index.blade.php

@@ -36,7 +36,9 @@
     @forelse($quizzes as $quiz)
     <tr class="hover:bg-slate-50">
       <td class="px-4 py-3 text-slate-600">{{ $quiz->subject->icon }} {{ $quiz->subject->name }}</td>
-      <td class="px-4 py-3 font-medium text-slate-800">{{ $quiz->title }}</td>
+      <td class="px-4 py-3 font-medium text-slate-800">{{ $quiz->title }}
+        @if($quiz->grade)<span class="ml-2 text-xs bg-indigo-100 text-indigo-700 px-2 py-0.5 rounded-full">Kl.{{ $quiz->grade }}</span>@endif
+      </td>
       <td class="px-4 py-3 text-center">
         <span class="{{ $quiz->questions_count < 10 ? 'text-amber-600' : 'text-green-600' }} font-medium">
           {{ $quiz->questions_count }}/10