feat(PROJ-12): E-Mail Export EML/PDF/ZIP

- GET /api/export/pdf/{id}: PDF-Generierung (stdlib, kein ext. Paket)
- POST /api/export/zip: Streaming-ZIP mit manifest.csv, Anhänge optional
- Max. 500 Mails pro Export, Zugriffscheck per Rolle
- Audit-Log für jeden Export
- Frontend: PDF-Button in Mail-Ansicht
- Frontend: Checkboxen + ZIP-Export-Dialog in Suchergebnissen

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/mail/[id]/page.tsx

@@ -6,6 +6,7 @@ import {
   getMail,
   downloadMailAttachment,
   downloadMailRaw,
+  exportMailPDF,
   type MailDetail,
   type MailAttachment,
 } from "@/lib/api";
@@ -227,6 +228,7 @@ export default function MailViewPage({
   const [error, setError] = useState<string | null>(null);
   const [loading, setLoading] = useState(true);
   const [downloading, setDownloading] = useState(false);
+  const [pdfLoading, setPdfLoading] = useState(false);
 
   useEffect(() => {
     if (!user) return;
@@ -250,6 +252,18 @@ export default function MailViewPage({
     }
   }
 
+  async function handlePdfDownload() {
+    setPdfLoading(true);
+    try {
+      const { blob, filename } = await exportMailPDF(id);
+      triggerDownload(blob, filename);
+    } catch (e) {
+      alert(`PDF-Export fehlgeschlagen: ${e instanceof Error ? e.message : e}`);
+    } finally {
+      setPdfLoading(false);
+    }
+  }
+
   if (authLoading || !user) {
     return (
       <div className="flex min-h-screen items-center justify-center">
@@ -282,6 +296,14 @@ export default function MailViewPage({
               >
                 {downloading ? "..." : "Als .eml herunterladen"}
               </Button>
+              <Button
+                variant="outline"
+                size="sm"
+                onClick={handlePdfDownload}
+                disabled={pdfLoading}
+              >
+                {pdfLoading ? "..." : "Als PDF exportieren"}
+              </Button>
             </div>
           )}
         </div>