fix: Render ProfileModal in App.jsx and support guest display name editing from Navbar

frontend/src/components/ProfileModal.jsx

@@ -4,7 +4,9 @@ import { apiFetch } from '../api/client';
 import { getRandomSkillColor } from '../utils/colors';
 
 export default function ProfileModal({ user, skills, onClose, onUserUpdated, onRefreshSkills }) {
-  const [displayName, setDisplayName] = useState(user?.display_name || '');
+  const [displayName, setDisplayName] = useState(
+    user?.display_name || localStorage.getItem('guest_display_name') || ''
+  );
   const [selectedSkillIds, setSelectedSkillIds] = useState(
     user?.skills ? user.skills.map(s => s.id) : []
   );
@@ -26,15 +28,22 @@ export default function ProfileModal({ user, skills, onClose, onUserUpdated, onR
     setSavingProfile(true);
 
     try {
-      const updatedUser = await apiFetch('/users/me/', {
-        method: 'PATCH',
-        body: JSON.stringify({
-          display_name: displayName.trim(),
-          skill_ids: selectedSkillIds
-        })
-      });
-      setSuccess('Profil und Anzeigename erfolgreich aktualisiert!');
-      onUserUpdated(updatedUser);
+      if (user) {
+        const updatedUser = await apiFetch('/users/me/', {
+          method: 'PATCH',
+          body: JSON.stringify({
+            display_name: displayName.trim(),
+            skill_ids: selectedSkillIds
+          })
+        });
+        setSuccess('Profil und Anzeigename erfolgreich aktualisiert!');
+        if (onUserUpdated) onUserUpdated(updatedUser);
+      } else {
+        localStorage.setItem('guest_display_name', displayName.trim());
+        window.dispatchEvent(new Event('guest_name_updated'));
+        setSuccess('Gast-Name erfolgreich gespeichert!');
+        setTimeout(() => onClose(), 600);
+      }
     } catch (err) {
       setError(err.message || 'Speichern des Profils fehlgeschlagen.');
     } finally {