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/utils/api-error.ts

@@ -2,7 +2,18 @@ import axios from 'axios';
 
 export function getApiErrorMessage(err: unknown, fallback: string): string {
   if (axios.isAxiosError(err)) {
-    return err.response?.data?.detail ?? fallback;
+    const detail = err.response?.data?.detail;
+    if (typeof detail === 'string') return detail;
+    // Pydantic V2 validation errors: [{type, loc, msg, input, ctx}]
+    if (Array.isArray(detail) && detail.length > 0) {
+      return detail
+        .map((e: { loc?: (string | number)[]; msg?: string }) => {
+          const field = e.loc?.filter((s) => s !== 'body').join('.') ?? '';
+          return field ? `${field}: ${e.msg}` : (e.msg ?? '');
+        })
+        .join('; ');
+    }
+    return fallback;
   }
   return fallback;
 }