/**
 * Prompts the user for a filename before download. Returns the chosen base
 * name (without extension) sanitized for filesystem use, or null if cancelled.
 *
 * @param defaultName Suggested base name (no extension)
 * @param ext Extension shown to the user, e.g. "pdf", "docx", "csv"
 */
export function promptFilename(defaultName: string, ext: string): string | null {
  const sanitize = (s: string) => s.replace(/[\\/:*?"<>|]+/g, "_").trim();
  const cleanDefault = sanitize(defaultName).replace(new RegExp(`\\.${ext}$`, "i"), "") || "Document";
  const input = typeof window !== "undefined"
    ? window.prompt(`File name (.${ext})`, cleanDefault)
    : cleanDefault;
  if (input === null) return null; // user cancelled
  const cleaned = sanitize(input).replace(new RegExp(`\\.${ext}$`, "i"), "");
  return cleaned || cleanDefault;
}