When a file URL is entered (e.g. https://xpra.org/src/xpra-6.4.3.tar.xz),
download it via curl through each gateway proxy in parallel and display
speed comparison. Highlights slow gateways (<10% of max) in red and
fast ones (>50% of max) in green.

Also add dark mode CSS support.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>