feat: Implement supervisor trainee management features, including search, add, and remove functionalities

Co-authored-by: Copilot <copilot@github.com>

generic/forms.py

@@ -835,6 +835,37 @@ class TraineeForm(Form):
 
 
 class SupervisorForm(ModelForm):
+    # UserSearchWidget returns list-like values; this field normalises to a
+    # single selected user for the Supervisor.user OneToOne relation.
+    user = ModelChoiceField(
+        queryset=User.objects.all(),
+        required=False,
+        widget=UserSearchWidget(),
+    )
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        user_field = self.fields.get("user")
+        if user_field:
+            user_field.widget = UserSearchWidget(user_field)
+
+    def clean_user(self):
+        value = self.cleaned_data.get("user")
+
+        if isinstance(value, (list, tuple)):
+            value = value[0] if value else None
+
+        if value is None:
+            return None
+
+        if isinstance(value, User):
+            return value
+
+        try:
+            return User.objects.get(pk=value)
+        except (TypeError, ValueError, User.DoesNotExist):
+            raise ValidationError("Select a valid user.")
+
     class Meta:
         model = Supervisor
         exclude = ("",)