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/Models/User.php

@@ -5,7 +5,7 @@ use Illuminate\Foundation\Auth\User as Authenticatable;
 use Illuminate\Notifications\Notifiable;
 class User extends Authenticatable {
     use HasFactory, Notifiable;
-    protected $fillable = ['name','email','password','role','points'];
+    protected $fillable = ['name','email','password','role','points','grade'];
     protected $hidden = ['password','remember_token'];
     protected $casts = ['email_verified_at' => 'datetime', 'password' => 'hashed'];