Handle ExtraBody clearing when frontend sends empty object

The backend now interprets an empty object sent from the frontend as a
signal to clear the ExtraBody field, while nil/undefined preserves the
existing value. Frontend changed to send {} instead of undefined when
the field is empty.

web/backend/api/models.go

@@ -202,8 +202,13 @@ func (h *Handler) handleUpdateModel(w http.ResponseWriter, r *http.Request) {
 	} else {
 		mc.ModelConfig.SetAPIKey(mc.APIKey)
 	}
+	// Preserve existing ExtraBody when omitted (nil), but clear it when
+	// the frontend sends an empty object {} to indicate the field should
+	// be removed.
 	if mc.ExtraBody == nil {
 		mc.ExtraBody = cfg.ModelList[idx].ExtraBody
+	} else if len(mc.ExtraBody) == 0 {
+		mc.ExtraBody = nil
 	}
 
 	cfg.ModelList[idx] = &mc.ModelConfig

web/frontend/src/components/models/edit-model-sheet.tsx

@@ -117,7 +117,7 @@ export function EditModelSheet({
         thinking_level: form.thinkingLevel || undefined,
         extra_body: form.extraBody.trim()
           ? JSON.parse(form.extraBody.trim())
-          : undefined,
+          : {},
       })
       if (setAsDefault && !model.is_default) {
         await setDefaultModel(model.model_name)