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/admin/pages/InvoiceDetail.tsx

@@ -615,6 +615,16 @@ export default function InvoiceDetail() {
           bank_account: inv.bank_account || "",
         });
 
+        // Calculate dueDays from existing dates
+        if (inv.issue_date && inv.due_date) {
+          const issue = new Date(inv.issue_date);
+          const due = new Date(inv.due_date);
+          const diffDays = Math.round(
+            (due.getTime() - issue.getTime()) / (1000 * 60 * 60 * 24),
+          );
+          if (diffDays > 0 && diffDays <= 60) setDueDays(diffDays);
+        }
+
         // Populate items from existing invoice
         if (inv.items?.length > 0) {
           setItems(
@@ -645,14 +655,13 @@ export default function InvoiceDetail() {
     if (isEdit) fetchDetail();
   }, [isEdit, fetchDetail]);
 
-  // ─── Create mode: due date calculation ───
+  // ─── Due date calculation from issue date + days ───
   useEffect(() => {
-    if (isEdit) return;
     if (!form.issue_date) return;
     const d = new Date(form.issue_date);
     d.setDate(d.getDate() + dueDays);
     setForm((prev) => ({ ...prev, due_date: d.toISOString().split("T")[0] }));
-  }, [isEdit, form.issue_date, dueDays]);
+  }, [form.issue_date, dueDays]);
 
   // ─── Create mode: customer filtering ───
   const filteredCustomers = useMemo(() => {
@@ -1540,37 +1549,25 @@ export default function InvoiceDetail() {
                   }}
                 />
               </FormField>
-              {!isEdit ? (
-                <FormField label="Splatnost (dny)">
-                  <select
-                    value={dueDays}
-                    onChange={(e) => setDueDays(Number(e.target.value))}
-                    className="admin-form-select"
-                  >
-                    {Array.from({ length: 60 }, (_, i) => i + 1).map((n) => (
-                      <option key={n} value={n}>
-                        {n}
-                      </option>
-                    ))}
-                  </select>
-                  {form.due_date && (
-                    <span className="text-tertiary text-xs mt-1">
-                      Splatnost:{" "}
-                      {new Date(form.due_date).toLocaleDateString("cs-CZ")}
-                    </span>
-                  )}
-                </FormField>
-              ) : (
-                <FormField label="Datum splatnosti">
-                  <AdminDatePicker
-                    mode="date"
-                    value={form.due_date}
-                    onChange={(val: string) => {
-                      setForm((prev) => ({ ...prev, due_date: val }));
-                    }}
-                  />
-                </FormField>
-              )}
+              <FormField label="Splatnost (dny)">
+                <select
+                  value={dueDays}
+                  onChange={(e) => setDueDays(Number(e.target.value))}
+                  className="admin-form-select"
+                >
+                  {Array.from({ length: 60 }, (_, i) => i + 1).map((n) => (
+                    <option key={n} value={n}>
+                      {n}
+                    </option>
+                  ))}
+                </select>
+                {form.due_date && (
+                  <span className="text-tertiary text-xs mt-1">
+                    Splatnost:{" "}
+                    {new Date(form.due_date).toLocaleDateString("cs-CZ")}
+                  </span>
+                )}
+              </FormField>
               <FormField label="DÚZP" error={errors.tax_date} required>
                 <AdminDatePicker
                   mode="date"