feat(ar1-admin): bottone Anteprima PDF wirato a BE /admin/ar1-templates/:id/preview-pdf

Rimosso toast 'TODO' dal bottone eye icon in Tab Template. Ora:
  1. chiama Ar1Service.previewTemplatePdf(row.id) (nuovo metodo, ritorna Blob)
  2. crea URL.createObjectURL + window.open('_blank')
  3. revokeObjectURL dopo 60s (cleanup)
  4. toast info iniziale 'Generazione anteprima...' + error toast su fail

Service: +1 metodo previewTemplatePdf(templateId) che torna Promise<Blob>
usando buildHeadersMultipart (nessun Content-Type per risposta binaria).

src/modules/ar1/service/ar1Service.js

@@ -210,6 +210,12 @@ const Ar1Service = {
     },
 
     // ---------- ADMIN: document categories (per dropdown) ----------
+    previewTemplatePdf(templateId) {
+        return fetch(`${BASE_URL}/admin/ar1-templates/${templateId}/preview-pdf`, {
+            method: 'POST', mode: 'cors', headers: buildHeadersMultipart()
+        }).then(r => { if (!r.ok) throw new Error(`HTTP ${r.status}`); return r.blob(); });
+    },
+
     listDocumentCategories(onSuccess, onError) {
         fetch(`${BASE_URL}/admin/document-categories`, {
             method: 'GET', mode: 'cors', headers: buildHeaders()