Enhance Destinataer functionality: inline editing and improved list view

- Add inline edit mode to destinataer detail view with AJAX save/cancel
- Fix form validation by aligning select choices with model definitions
- Update Destinataer model to make familienzweig and berufsgruppe optional
- Fix StiftungsKonto integration in forms and views
- Redesign destinataer list view with new column layout:
  * Vorname, Nachname, E-Mail, Vierteljährlicher Betrag
  * Letzter Studiennachweis, Unterstützung bestätigt, Aktionen
- Improve form styling and user experience
- Add proper field validation and error handling
- Enhance UI with better badges, icons, and formatting

app/stiftung/models.py

@@ -199,7 +199,7 @@ class Destinataer(models.Model):
 
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
     familienzweig = models.CharField(
-        max_length=100, choices=FAMILIENZWIG_CHOICES, default="hauptzweig"
+        max_length=100, choices=FAMILIENZWIG_CHOICES, blank=True, null=True
     )
     vorname = models.CharField(max_length=100, verbose_name="Vorname")
     nachname = models.CharField(max_length=100, verbose_name="Nachname")
@@ -221,7 +221,8 @@ class Destinataer(models.Model):
     berufsgruppe = models.CharField(
         max_length=20,
         choices=BERUFSGRUPPE_CHOICES,
-        default="andere",
+        blank=True,
+        null=True,
         verbose_name="Berufsgruppe",
     )
     ausbildungsstand = models.CharField(