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/Offers.tsx

@@ -221,21 +221,27 @@ export default function Offers() {
 
   const handlePdf = async (quotation: Quotation) => {
     if (pdfLoading) return;
+    const newWindow = window.open("", "_blank");
     setPdfLoading(quotation.id);
     try {
       const response = await apiFetch(
         `${API_BASE}/offers/${quotation.id}/file`,
       );
-      if (response.status === 401) return;
+      if (response.status === 401) {
+        newWindow?.close();
+        return;
+      }
       if (!response.ok) {
+        newWindow?.close();
         alert.error("PDF soubor nenalezen — otevřete nabídku a uložte ji");
         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 připojení");
     } finally {
       setPdfLoading(null);