fix: OfferDetail uses default currency from system settings

The useEffect checked prev.currency === "EUR" but initial default was
changed to "CZK", so the settings default was never applied.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin/pages/OfferDetail.tsx

@@ -356,7 +356,7 @@ export default function OfferDetail() {
       setForm((prev) => ({
         ...prev,
         currency:
-          prev.currency === "EUR"
+          prev.currency === "CZK"
             ? companySettings.default_currency || "CZK"
             : prev.currency,
         vat_rate: