/**
 * Resolves a favicon URL for a given domain.
 *
 * On Chrome MV3 with the `favicon` permission, returns a URL backed by the
 * browser's own favicon cache (no network, no third party). On Firefox and
 * other browsers where the API is unavailable, returns null so callers can
 * render a generic fallback icon.
 *
 * Never fetches from any external URL. No domain name leaves the browser.
 */
export function faviconUrl(domain: string): string | null {
  if (!domain) return null;
  if (typeof chrome === "undefined" || !chrome.runtime?.getURL) return null;

  try {
    const url = new URL(chrome.runtime.getURL("/_favicon/"));
    url.searchParams.set("pageUrl", `https://${domain}`);
    url.searchParams.set("size", "32");
    return url.toString();
  } catch {
    return null;
  }
}