fix(show-me): keep the header's counters and keys on one line

[arasovic]

On a narrow pane, every part of the show-me header could shrink or wrap. "turn 1/2" could break over two lines, and a long title could push the ‹ › open close keys out of view.

• The counters ("1/3", "turn 1/2") and the keys now hold their width. Only the title gives way, cut with an ellipsis instead of wrapping.
• The header is the same on every surface, so the terminal gets this too.
• A new test, run on the terminal and on desktop, checks that the title truncates and that the counters and keys sit in boxes that do not shrink. It fails on the previous code.
• Version 0.4.7.