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

app/Http/Controllers/Child/QuizController.php

@@ -8,7 +8,12 @@ class QuizController extends Controller {
 
     public function index() {
         $subjects = Subject::all()->keyBy('id');
+        $grade    = auth()->user()->grade;
         $quizzes  = Quiz::with('subject')->where('active',true)
+            ->where(function($q) use ($grade) {
+                $q->whereNull('grade');
+                if ($grade) $q->orWhere('grade', $grade);
+            })
             ->withCount('questions')->get()->groupBy('subject_id');
         $bestScores = QuizAttempt::where('user_id',auth()->id())
             ->where('status','completed')