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/users/index.blade.php

@@ -11,6 +11,7 @@
       <tr>
         <th class="text-left px-4 py-3 font-medium text-slate-600">Name</th>
         <th class="text-left px-4 py-3 font-medium text-slate-600">E-Mail</th>
+        <th class="text-center px-4 py-3 font-medium text-slate-600">Klasse</th>
         <th class="text-right px-4 py-3 font-medium text-slate-600">🪙 Münzen</th>
         <th class="text-right px-4 py-3 font-medium text-slate-600">Antworten</th>
         <th class="px-4 py-3"></th>
@@ -21,7 +22,8 @@
       <tr class="hover:bg-slate-50">
         <td class="px-4 py-3 font-medium text-slate-800">{{ $u->name }}</td>
         <td class="px-4 py-3 text-slate-500">{{ $u->email }}</td>
-        <td class="px-4 py-3 text-right font-bold text-amber-600">{{ $u->points }}</td>
+        <td class="px-4 py-3 text-center text-sm text-slate-500">{{ $u->grade ? "Klasse ".$u->grade : "–" }}</td>
+      <td class="px-4 py-3 text-right font-bold text-amber-600">{{ $u->points }}</td>
         <td class="px-4 py-3 text-right text-slate-500">{{ $u->attempts_count }}</td>
         <td class="px-4 py-3 text-right whitespace-nowrap">
           <a href="{{ route('admin.users.edit',$u) }}" class="text-violet-600 hover:underline mr-3">Bearbeiten</a>