fix: file viewers blocked on mobile — open blank window before async fetch

Mobile browsers block window.open() after async operations. Changed all
file viewers to open a blank window synchronously in the click handler,
then set location.href after fetch completes.

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

src/admin/pages/OfferDetail.tsx

@@ -768,19 +768,25 @@ export default function OfferDetail() {
 
   const handlePdf = async () => {
     if (!isEdit || pdfLoading) return;
+    const newWindow = window.open("", "_blank");
     setPdfLoading(true);
     try {
       const response = await apiFetch(`${API_BASE}/offers/${id}/file`);
-      if (response.status === 401) return;
+      if (response.status === 401) {
+        newWindow?.close();
+        return;
+      }
       if (!response.ok) {
+        newWindow?.close();
         alert.error("PDF soubor nenalezen — uložte nabídku pro vygenerování");
         return;
       }
       const blob = await response.blob();
       const url = URL.createObjectURL(blob);
-      window.open(url, "_blank");
+      if (newWindow) newWindow.location.href = url;
       setTimeout(() => URL.revokeObjectURL(url), 60000);
     } catch {
+      newWindow?.close();
       alert.error("Chyba při generování PDF");
     } finally {
       setPdfLoading(false);