fix: convert dates to yyyy-MM-dd when opening received invoice edit form

ISO datetime strings from API caused "Invalid time value" in date picker.

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

src/admin/pages/ReceivedInvoices.tsx

@@ -302,11 +302,20 @@ export default function ReceivedInvoices({ statsMonth, statsYear, uploadOpen, se
   }
 
   // Edit handlers
+  const toDateInput = (d: string | null | undefined): string => {
+    if (!d) return ''
+    const date = new Date(d)
+    if (isNaN(date.getTime())) return ''
+    return date.toISOString().split('T')[0]
+  }
+
   const openEdit = (inv: ReceivedInvoice) => {
     setEditInvoice({
       ...inv,
       amount: String(inv.amount),
       vat_rate: String(inv.vat_rate),
+      issue_date: toDateInput(inv.issue_date),
+      due_date: toDateInput(inv.due_date),
       _originalStatus: inv.status,
     })
     setEditOpen(true)