/**
 * Downscale an image file to a thumbnail and return an object URL.
 * Returns undefined for non-image files.
 * Caller is responsible for revoking the URL via URL.revokeObjectURL().
 */
export async function createImageThumbnail(
  file: File,
  maxDim = 200,
): Promise<string | undefined> {
  if (!file.type.startsWith("image/")) return undefined;

  const bitmap = await createImageBitmap(file);
  const scale = Math.min(1, maxDim / Math.max(bitmap.width, bitmap.height));
  const w = Math.round(bitmap.width * scale);
  const h = Math.round(bitmap.height * scale);

  const canvas = new OffscreenCanvas(w, h);
  const ctx = canvas.getContext("2d")!;
  ctx.drawImage(bitmap, 0, 0, w, h);
  bitmap.close();

  const blob = await canvas.convertToBlob({ type: "image/jpeg", quality: 0.7 });
  return URL.createObjectURL(blob);
}