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/ReceivedInvoices.tsx

@@ -503,19 +503,22 @@ export default function ReceivedInvoices({
   };
 
   const openFile = async (inv: ReceivedInvoice) => {
+    const newWindow = window.open("", "_blank");
     try {
       const response = await apiFetch(
         `${API_BASE}/received-invoices/${inv.id}/file`,
       );
       if (!response.ok) {
+        newWindow?.close();
         alert.error("Nepodařilo se načíst soubor");
         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");
     }
   };