Instructions site offers no answer to a Mac user

index.html asks one question — Windows or Linux — because those are the two installers that exist. A Mac user reaching the site currently finds no answer at all, not even "not yet".

Depends on #11 / #8. Two levels of fix:

• If Mac support is not happening soon: add a line under the two cards saying so, so a Mac user knows to stop looking rather than assuming they picked the wrong page.
• If a Mac installer lands: add a third card and a mac.html built from the same four steps. The page is a copy of linux.html with different specifics; the CSS grid already reflows to three cards with no change.

Same for the help.html question "whether you are on Windows or Linux", and the ?os= parameter that the token page uses to send people back to the right page — pick_os() in server/register.py would need "mac" added to its allowed set.

Start by checking the status of #11 and #8, then compare docs/index.html, help.html, linux.html, and server/register.py, especially pick_os(). Done means Mac visitors receive either an explicit unsupported message or a complete Mac path, including the card, mac.html, and ?os= routing if an installer exists.