Show error toast when leads API fails in Leadspeicher

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

app/leadspeicher/page.tsx

@@ -443,6 +443,9 @@ export default function LeadVaultPage() {
         setLeads(data.leads);
         setTotal(data.total);
         setPages(data.pages);
+      } else {
+        const err = await res.json().catch(() => ({})) as { error?: string };
+        toast.error(`Leads konnten nicht geladen werden: ${err.error || res.status}`);
       }
     } finally {
       setLoading(false);