Reapply "Add missing admin action files and navigation links"

This reverts commit 4d515bc400.

src/actions/admin-vouchers.ts

@@ -9,7 +9,9 @@ import { logServerError } from "@/lib/server-log";
 export async function createVoucher(formData: FormData): Promise<void> {
   await requireStaff();
 
-  const code = String(formData.get("code") ?? "").trim().slice(0, 255);
+  const code = String(formData.get("code") ?? "")
+    .trim()
+    .slice(0, 255);
   const amount = Number(formData.get("amount"));
   const maxUsesRaw = Number(formData.get("maxUses"));
   const maxUses = Number.isFinite(maxUsesRaw) && maxUsesRaw > 0 ? Math.floor(maxUsesRaw) : 1;