Download page: extension install guide for Chrome and Firefox ...

- public /download page (nav item «Расширение»): per-browser download
  buttons and step-by-step install instructions, setup (server URL is
  the panel's own origin) and hotkey reference, en/ru
- npm run package -w @ltt/extension zips dist/{chrome,firefox} into
  packages/web/public/ext (stable names, manifest at archive root so
  "Load unpacked" works right after unpacking)
- make ext / make prod always build fresh zips; deploy/.env.example
  seeds the release address bugtrail.gnexus.space

Co-Authored-By: Claude Code <noreply@anthropic.com>