feat: reminder due calculation

app/vouchers/new/page.tsx

@@ -23,6 +23,7 @@ export default function NewVoucherPage() {
   const [serverError, setServerError] = useState("");
   const [contacts, setContacts] = useState<{ id: number; name: string }[]>([]);
   const [contactIds, setContactIds] = useState<number[]>([]);
+  const [reminderDays, setReminderDays] = useState("30,6,1");
 
   useEffect(() => {
     fetch("/api/contacts")
@@ -55,7 +56,7 @@ export default function NewVoucherPage() {
     const res = await fetch("/api/vouchers", {
       method: "POST",
       headers: { "content-type": "application/json" },
-      body: JSON.stringify({ ...input, contactIds }),
+      body: JSON.stringify({ ...input, contactIds, reminderDays }),
     });
     if (!res.ok) {
       const data = await res.json().catch(() => ({}));
@@ -87,6 +88,20 @@ export default function NewVoucherPage() {
             {errors.includes(f.name) && <p className="mt-1 text-sm text-red-600">Ungültig: {f.name}</p>}
           </div>
         ))}
+        <div>
+          <label htmlFor="reminderDays" className="block text-sm font-medium">
+            Erinnerung (Tage vorher, Kommaliste)
+          </label>
+          <input
+            id="reminderDays"
+            name="reminderDays"
+            type="text"
+            value={reminderDays}
+            onChange={(e) => setReminderDays(e.target.value)}
+            placeholder="30,6,1"
+            className="mt-1 w-full rounded border px-3 py-2"
+          />
+        </div>
         <section id="kontakte">
           <h2 className="text-sm font-medium">Empfänger</h2>
           {contacts.map((c) => (