// Supported clients: iOS Safari, Android Chrome, PC Chrome/Edge/Firefox.
// Gated on required API presence; anything missing → hard stop.
export function supported(): string | null {
  if (typeof window === 'undefined') return 'window unavailable';
  if (typeof createImageBitmap !== 'function') return 'createImageBitmap';
  if (typeof Worker === 'undefined') return 'Worker';
  if (typeof indexedDB === 'undefined') return 'IndexedDB';
  const hasOffscreen =
    typeof OffscreenCanvas !== 'undefined' &&
    typeof OffscreenCanvas.prototype.convertToBlob === 'function';
  const hasCanvasBlob =
    typeof HTMLCanvasElement !== 'undefined' &&
    typeof HTMLCanvasElement.prototype.toBlob === 'function';
  if (!hasOffscreen && !hasCanvasBlob) return 'canvas PNG encode';
  const ua = navigator.userAgent;
  const isIOS = /iPad|iPhone|iPod/.test(ua) || (navigator.platform === 'MacIntel' && navigator.maxTouchPoints > 1);
  const isAndroid = /Android/i.test(ua);
  const isSafari = /Safari/i.test(ua) && !/Chrome|CriOS|Edg|Firefox|FxiOS/i.test(ua);
  const isChrome = /Chrome|CriOS/i.test(ua) && !/Edg/i.test(ua);
  const isEdge = /Edg/i.test(ua);
  const isFirefox = /Firefox|FxiOS/i.test(ua);
  if (isIOS && isSafari) return null;
  if (isAndroid && isChrome) return null;
  if (!isIOS && !isAndroid && (isChrome || isEdge || isFirefox)) return null;
  return 'unsupported browser';
}