PNG conversion strips GIF animation. GIFs are now detected by URL
pattern (.gif extension or Tenor/Klipy CDN) and copied as URL text
instead, so pasting back into chat re-renders the animated GIF.
Implements image save/download (blob fetch + anchor trigger, fallback to window.open)
and clipboard copy (PNG write via ClipboardItem, fallback to URL text copy) with
toast feedback. Also adds ClipboardItem polyfill and Response.blob() content-type
fix to the jsdom test setup so the clipboard test suite runs correctly.