Boards whose columns are not labels

Not every GitLab board is built from labels. GitLab lets a list be defined by an assignee or a milestone, and a board mixing those with label lists was previously rendering only part of itself.

Those lists now render.

Two related fixes came with it, and the second matters more than it looks:

• Moving a card into a non-label column no longer causes the card to vanish from the board on the next fetch.
• A board that still has lists we cannot draw now tells you so on the board, naming what is missing.

That second one is the point. The old behaviour was not "unsupported", it was "silently incomplete" — a board that looked whole and was not, with no way to tell the difference from the outside. A tool that cannot show you something should say which thing, not present a partial answer as a full one.