fix: invoice edit/list improvements

- Due date uses days selector in edit mode (same as create)
- Overdue invoices fully editable (same as issued)
- Overdue status reversed to issued when due date moved to future
- Invoice list: edit icon for issued/overdue, eye for paid
- Invoice list: PDF opens blob from NAS (removed lang modal)
- NAS cleanup: properly scans directories when cleaning old PDFs
- Fixed syntax error from leftover else block

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

src/services/invoices.service.ts

@@ -70,6 +70,11 @@ export async function markOverdueInvoices() {
       where: { status: "issued", due_date: { lt: new Date() } },
       data: { status: "overdue" },
     });
+    // Reverse: if due_date was changed to future, set back to issued
+    await prisma.invoices.updateMany({
+      where: { status: "overdue", due_date: { gte: new Date() } },
+      data: { status: "issued" },
+    });
   } catch (err) {
     console.error("markOverdueInvoices failed:", err);
   }
@@ -350,8 +355,8 @@ export async function updateInvoice(id: number, body: Record<string, any>) {
 
   const data: Record<string, unknown> = { modified_at: new Date() };
 
-  // Only allow full editing in 'issued' state
-  const isDraft = currentStatus === "issued";
+  // Allow full editing in 'issued' and 'overdue' states
+  const isDraft = currentStatus === "issued" || currentStatus === "overdue";
   if (isDraft) {
     const strFields = [
       "currency",