fix: Export-Encoding und Excel-Support

- CSV: UTF-8 BOM + \r\n Zeilenenden → Umlaute in Excel korrekt
- CSV: deutsche Spaltennamen, Tags als kommagetrennte Liste
- Excel (.xlsx): nativer Export via xlsx-Library mit Spaltenbreiten
- Export-Dropdown: CSV und Excel jeweils für aktuelle Ansicht und nur mit E-Mail

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

app/leadvault/page.tsx

@@ -520,8 +520,8 @@ export default function LeadVaultPage() {
     }
   }
 
-  function exportCSV(emailOnly = false) {
-    const params = new URLSearchParams({ search: debouncedSearch, sortBy, sortDir });
+  function exportFile(format: "csv" | "xlsx", emailOnly = false) {
+    const params = new URLSearchParams({ search: debouncedSearch, sortBy, sortDir, format });
     filterStatus.forEach(s => params.append("status", s));
     filterSource.forEach(s => params.append("sourceTab", s));
     if (filterHasEmail) params.set("hasEmail", filterHasEmail);
@@ -560,9 +560,10 @@ export default function LeadVaultPage() {
               </button>
               <div className="absolute right-0 top-9 hidden group-hover:block z-50 bg-[#1a1a28] border border-[#2e2e3e] rounded-lg shadow-xl p-1 min-w-[220px]">
                 {[
-                  ["Aktuelle Ansicht (CSV)", () => exportCSV()],
-                  ["Alle Leads (CSV)", () => exportCSV()],
-                  ["Nur mit E-Mail (CSV)", () => exportCSV(true)],
+                  ["Aktuelle Ansicht (CSV)", () => exportFile("csv")],
+                  ["Aktuelle Ansicht (Excel)", () => exportFile("xlsx")],
+                  ["Nur mit E-Mail (CSV)", () => exportFile("csv", true)],
+                  ["Nur mit E-Mail (Excel)", () => exportFile("xlsx", true)],
                 ].map(([label, fn]) => (
                   <button key={label as string} onClick={fn as () => void}
                     className="w-full text-left px-3 py-2 text-sm text-gray-300 hover:bg-[#2e2e3e] rounded">