fix: received invoices — show skeleton only on initial load, not on sort/filter

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

src/admin/pages/ReceivedInvoices.tsx

@@ -155,7 +155,7 @@ export default function ReceivedInvoices({ statsMonth, statsYear, uploadOpen, se
 
   // Fetch list
   const fetchList = useCallback(async () => {
-    setLoading(true)
+    if (!hasLoadedOnce.current) setLoading(true)
     try {
       const params = new URLSearchParams({
         month: String(statsMonth),
@@ -171,6 +171,7 @@ export default function ReceivedInvoices({ statsMonth, statsYear, uploadOpen, se
       }
     } catch { /* ignore */ } finally {
       setLoading(false)
+      hasLoadedOnce.current = true
     }
   }, [statsMonth, statsYear, search, sort, order])