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/LearnController.php

@@ -22,14 +22,24 @@ class LearnController extends Controller {
         $answeredToday = QuestionAttempt::where('user_id',$user->id)
             ->whereDate('created_at', today())
             ->pluck('question_id');
+        $grade    = $user->grade;
         $question = $subject->activeQuestions()
             ->whereNotIn('id', $answeredToday)
+            ->where(function($q) use ($grade) {
+                $q->whereNull('grade');
+                if ($grade) $q->orWhere('grade', $grade);
+            })
             ->with('answerOptions')
             ->inRandomOrder()
             ->first();
         // Falls alle beantwortet: ganz random
         if (!$question) {
-            $question = $subject->activeQuestions()->with('answerOptions')->inRandomOrder()->first();
+            $question = $subject->activeQuestions()
+                ->where(function($q) use ($grade) {
+                    $q->whereNull('grade');
+                    if ($grade) $q->orWhere('grade', $grade);
+                })
+                ->with('answerOptions')->inRandomOrder()->first();
         }
         if (!$question) {
             return redirect()->route('learn.subjects')->with('info','Noch keine Fragen für dieses Fach.');