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/nas-financials-manager.ts

@@ -41,7 +41,13 @@ class NasFinancialsManager {
         const yearPath = path.join(issuedDir, yearDir);
         if (!fs.statSync(yearPath).isDirectory()) continue;
         for (const monthDir of fs.readdirSync(yearPath)) {
-          const filePath = path.join(yearPath, monthDir, safeName);
+          const monthPath = path.join(yearPath, monthDir);
+          try {
+            if (!fs.statSync(monthPath).isDirectory()) continue;
+          } catch {
+            continue;
+          }
+          const filePath = path.join(monthPath, safeName);
           if (fs.existsSync(filePath)) {
             fs.unlinkSync(filePath);
           }