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/Admin/UserController.php

@@ -15,6 +15,7 @@ class UserController extends Controller {
             'name'     => 'required|string|max:60',
             'email'    => 'required|email|unique:users',
             'password' => 'required|min:6',
+            'grade'    => 'nullable|integer|min:1|max:13',
         ]);
         User::create([
             'name'     => $r->name,
@@ -22,6 +23,7 @@ class UserController extends Controller {
             'password' => Hash::make($r->password),
             'role'     => 'child',
             'points'   => 0,
+            'grade'    => $r->grade ?: null,
         ]);
         return redirect()->route('admin.users.index')->with('success','Kind-Konto erstellt.');
     }
@@ -32,8 +34,9 @@ class UserController extends Controller {
             'email'    => 'required|email|unique:users,email,'.$user->id,
             'password' => 'nullable|min:6',
             'points'   => 'required|integer|min:0',
+            'grade'    => 'nullable|integer|min:1|max:13',
         ]);
-        $user->fill(['name'=>$r->name,'email'=>$r->email,'points'=>$r->points]);
+        $user->fill(['name'=>$r->name,'email'=>$r->email,'points'=>$r->points,'grade'=>$r->grade ?: null]);
         if ($r->filled('password')) $user->password = Hash::make($r->password);
         $user->save();
         return redirect()->route('admin.users.index')->with('success','Gespeichert.');