Improve user search, DB nulls, and chat UI

Change username filter to use icontains for partial/case-insensitive matching. In CustomUser.save, introduce _NULLABLE_CHAR_FIELDS and convert empty string char fields to None to ensure nullable fields are stored consistently. Update CreateChatModal to always show a subtitle combining the user's full name and account creation date (joined with " · "), improving UI information.

backend/account/models.py

@@ -116,7 +116,13 @@ class CustomUser(SoftDeleteModel, AbstractUser):
 
         return super().delete(*args, **kwargs)
     
+    _NULLABLE_CHAR_FIELDS = ('phone_number', 'city', 'street', 'country', 'postal_code', 'email_verification_token')
+
     def save(self, *args, **kwargs):
+        for field in self._NULLABLE_CHAR_FIELDS:
+            if not getattr(self, field):
+                setattr(self, field, None)
+
         is_new = self._state.adding  # True if object hasn't been saved yet
 
         # Pre-save flags for new users