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/create.blade.php

@@ -22,6 +22,15 @@
         <input name="password" type="password" required class="w-full border border-slate-300 rounded-lg px-3 py-2 text-sm focus:ring-2 focus:ring-violet-500 focus:border-transparent outline-none">
         @error('password')<p class="text-red-500 text-xs mt-1">{{ $message }}</p>@enderror
       </div>
+      <div>
+        <label class="block text-sm font-medium text-slate-700 mb-1">Klasse</label>
+        <select name="grade" class="w-full border border-slate-300 rounded-lg px-3 py-2 text-sm focus:ring-2 focus:ring-violet-500 outline-none">
+          <option value="">– Keine Klasse –</option>
+          @for($g=1;$g<=10;$g++)
+          <option value="{{ $g }}" {{ old('grade','')==$g?'selected':'' }}>Klasse {{ $g }}</option>
+          @endfor
+        </select>
+      </div>
       <button type="submit" class="w-full bg-violet-600 hover:bg-violet-700 text-white py-2 rounded-lg font-medium text-sm">Konto erstellen</button>
     </form>
   </div>