feat: dodaj ishodište i PDF račune za putni nalog

Dodano je origin_location polje kroz backend model, serializer, admin i UI
modal za kreiranje/uređivanje putnog naloga (default: Zagreb).

Računi putnog naloga sada prihvaćaju slike i PDF datoteke, uz backend
validaciju formata, podršku za serving PDF-a i prilagodbu preview prikaza.

Za greške uploada računa dodan je error toast kako bi poruke validacije bile
vidljive korisniku odmah u sučelju.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/components/dashboard/WorkOrderInvoicesPdfPage.jsx

@@ -23,8 +23,22 @@ function readWorkOrderIdFromQuery() {
     return params.get('workOrderId') || '';
 }
 
-function InvoiceImagePreview({ imageUrl, alt }) {
+function InvoiceImagePreview({ imageUrl, alt, contentType }) {
     const [broken, setBroken] = useState(false);
+    const isPdf = String(contentType || '').toLowerCase() === 'application/pdf';
+
+    if (isPdf && imageUrl) {
+        return (
+            <a
+                href={imageUrl}
+                target="_blank"
+                rel="noopener noreferrer"
+                className="flex h-28 w-28 items-center justify-center rounded-lg border border-border-hairline bg-canvas-deep text-center text-[11px] text-text-main hover:bg-canvas-base"
+            >
+                Otvori PDF
+            </a>
+        );
+    }
 
     if (!imageUrl || broken) {
         return (
@@ -307,6 +321,7 @@ export default function WorkOrderInvoicesPdfPage() {
                                     <InvoiceImagePreview
                                         imageUrl={getAuthenticatedMediaSrc(invoice.image_url || invoice.image) || resolveMediaUrl(invoice.image_url || invoice.image)}
                                         alt={`Račun ${invoice.naziv_racuna}`}
+                                        contentType={invoice.image_content_type}
                                     />
                                 </div>
                             </article>