fix: dohvat mjesecnih racuna za troskove

Uklanja ovisnost o workOrders.invoices i dohvaća račune direktno kroz API filtriran po mjesecu. Dodaje i display code putnog naloga u serializer kako bi prikaz u izvještaju ostao potpun.

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

frontend/src/stores/fleetDashboardStore.js

@@ -920,6 +920,13 @@ export async function fetchMonthlyServicerEntries(year, month) {
     return Array.isArray(payload) ? payload : (payload?.results ?? []);
 }
 
+export async function fetchMonthlyCostInvoices(year, month) {
+    const payload = await api.get(
+        `fleet/work-order-invoices/?year=${year}&month=${month}`
+    );
+    return Array.isArray(payload) ? payload : (payload?.results ?? []);
+}
+
 export async function upsertMonthlyServicerEntry(data = {}) {
     if (!data.entry_date) {
         throw new Error('Datum dnevnog unosa je obavezan.');