fix: handle Pydantic validation errors in notify + nullify empty optional fields

- getApiErrorMessage now formats Pydantic [{loc, msg}] arrays as readable strings
- LandingPaymentMethod description/icon_url nullable
- Send null instead of empty string for optional payment method fields

src/pages/AdminLandingEditor.tsx

@@ -406,7 +406,11 @@ export default function AdminLandingEditor() {
 
     // Strip _id before sending to API
     const cleanFeatures: AdminLandingFeature[] = features.map(({ _id: _, ...rest }) => rest);
-    const cleanMethods = paymentMethods.map(({ _id: _, ...rest }) => rest);
+    const cleanMethods = paymentMethods.map(({ _id: _, ...rest }) => ({
+      ...rest,
+      description: rest.description || null,
+      icon_url: rest.icon_url || null,
+    }));
 
     // Filter out empty period arrays and periods for non-selected tariffs
     const cleanPeriods = Object.fromEntries(