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/questions/index.blade.php

@@ -67,6 +67,7 @@
     <thead class="bg-slate-50 border-b border-slate-200">
       <tr>
         <th class="text-left px-4 py-3 font-medium text-slate-600">Frage</th>
+        <th class="text-center px-4 py-3 font-medium text-slate-600">Klasse</th>
         <th class="text-center px-4 py-3 font-medium text-slate-600">Schwierigkeit</th>
         <th class="text-center px-4 py-3 font-medium text-slate-600 hidden sm:table-cell">Aktiv</th>
         <th class="px-4 py-3"></th>
@@ -78,6 +79,7 @@
       <td class="px-4 py-3 text-slate-700 max-w-xs">
         <span class="line-clamp-2">{{ $q->question_text }}</span>
       </td>
+      <td class="px-4 py-3 text-center text-xs text-slate-500">{{ $q->grade ? "Kl.".$q->grade : "–" }}</td>
       <td class="px-4 py-3 text-center">{{ $q->difficultyStars() }}</td>
       <td class="px-4 py-3 text-center hidden sm:table-cell">{{ $q->active ? '✅' : '⏸️' }}</td>
       <td class="px-4 py-3 text-right whitespace-nowrap">