feat(contracts): preview PDF before sending

Add a 'Preview PDF' button on draft contracts that renders a fresh PDF
via the existing no-write /preview endpoint, so the admin can check
layout + signature blocks before sending (no audit trail created).

frontend/src/i18n/locales/de.json

@@ -4153,6 +4153,7 @@
       "new": "Neuer Vertrag"
     },
     "detail": {
+      "previewPdf": "PDF-Vorschau",
       "integrity": {
         "title": "PDF-Integritätsprüfung",
         "help": "Berechnet die SHA-256-Hashes der unsignierten und signierten PDFs neu und vergleicht sie mit den beim Ausstellen gespeicherten Werten. Deckt Backup-Beschädigungen oder nachträgliche Änderungen an der Kundenkopie auf.",

frontend/src/i18n/locales/en.json

@@ -4151,6 +4151,7 @@
       "new": "New contract"
     },
     "detail": {
+      "previewPdf": "Preview PDF",
       "integrity": {
         "title": "PDF integrity check",
         "help": "Re-hashes the unsigned + signed PDFs on disk and compares them to the SHA-256 stored when the document was issued. Catches backup corruption or manual edits since the customer received their copy.",

frontend/src/pages/admin/contracts/ContractDetailPage.tsx

@@ -197,6 +197,25 @@ export const ContractDetailPage: React.FC = () => {
     }
   }
 
+  // Pre-send preview: renders a fresh PDF from the current draft without
+  // writing/sending anything, so the admin can sanity-check layout +
+  // signature blocks before committing to send (no audit trail created).
+  async function handlePdfPreview() {
+    if (!numericId) return;
+    const previewWindow = window.open('about:blank', '_blank');
+    if (!previewWindow) {
+      toast.error(t('contracts.detail.popupBlocked', 'Allow pop-ups for this site to preview the PDF.') as string);
+      return;
+    }
+    try {
+      const url = await contractsService.previewPdfUrl(numericId);
+      previewWindow.location.href = url;
+    } catch (err: any) {
+      previewWindow.close();
+      toast.error(err?.response?.data?.error || 'Preview failed');
+    }
+  }
+
   async function handleSignedPdfDownload() {
     if (!numericId) return;
     const previewWindow = window.open('about:blank', '_blank');
@@ -241,6 +260,10 @@ export const ContractDetailPage: React.FC = () => {
               <Edit2 className="w-4 h-4 mr-1" />
               {t('contracts.detail.edit', 'Edit')}
             </Button>
+            <Button variant="outline" onClick={handlePdfPreview}>
+              <FileDown className="w-4 h-4 mr-1" />
+              {t('contracts.detail.previewPdf', 'Preview PDF')}
+            </Button>
             <Button onClick={() => sendMutation.mutate()} disabled={sendMutation.isPending}>
               <Send className="w-4 h-4 mr-1" />
               {t('contracts.detail.send', 'Send to customer')}