Get Chromium set up on Linux kiosk stations

A Linux station without Chromium showed the operator UI in a regular browser tab, with nothing telling you why or how to fix it.

• Added a check during station setup: when the kiosk UI is on and Chromium is missing, setup prints the install command for your distribution and offers to run it.
• Added a warning under the Kiosk UI setting on the station page while the station reports no Chromium.
• Updated the kiosk UI docs with install commands for Ubuntu, Debian, Raspberry Pi OS, Fedora, RHEL, Arch and openSUSE.