browser setup assumes a headed environment (xdg-open + interactive install prompt fails on headless)

On headless Linux (no DISPLAY/WAYLAND_DISPLAY), jcode browser setup xdg-opens the XPI and waits for an interactive Firefox "Add extension?" click, so it always times out with "Extension not detected".

Fix implemented and validated on headless Debian 13: sideload the signed bridge XPI into a managed profile (~/.jcode/browser/firefox-profile, extensions.autoDisableScopes=0) and start a detached headless Firefox. The repair path also restarts the managed headless Firefox when the bridge stops responding.

Branch (PR creation is blocked for my account on this repo): https://github.com/taylor-shift/jcode/tree/headless-browser-setup
Commit: https://github.com/taylor-shift/jcode/commit/64ec0785

Validation: fresh install connects, repair reconnects after killing Firefox, rerun is idempotent, bridge navigation works, cargo test -p jcode-base browser passes (2 new tests), clippy clean.

Start with the jcode browser setup command and the browser-related code covered by cargo test -p jcode-base browser; reproduce the failure on headless Linux without DISPLAY or WAYLAND_DISPLAY. Done means fresh setup connects, repair reconnects after Firefox stops, reruns are idempotent, and bridge navigation works with the browser tests and clippy passing.