Semester 1.23

Fixes the dashboard on Windows.

• Windows: the dashboard couldn't load. Dates were formatted with codes Windows rejects, so building the page failed with "invalid format string" — on every launch, for everyone on Windows. Sign-in looked broken for the same reason: it created and saved the token fine, then handed off to a page that couldn't render.
• Checks now run on Windows, Linux and macOS before every build, and each built app is launched and made to draw a dashboard before it ships.