The nav's feed link already points at the JSON feed, but was using
the feather RSS icon. Adds an SVG recreation of the official JSON
Feed mark (traced from jsonfeed.org's icon) and swaps it in, using a
single dark-green fill throughout so it stays legible against both
the light and dark theme backgrounds.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXyzNvQZPQgBoSZwW7cLG8
