Google Chrome blocks --load-extension, so CF tokens stay empty under
Xvfb. Auto-select Chromium when the extension is present, install/
detect chromium in start.sh, and document browser_prefer overrides.
Enable Chromium on machines without DISPLAY: Python auto headless
with Xvfb hints, and start.sh installs/starts Xvfb (xvfb-run or
persistent :99) so registration can run headed under a virtual display.