Show check, checkmate, and whose turn it is on the board

[wsams]

Summary

On a phone, check and checkmate were easy to miss. The status line sat above the clocks and the board, so looking at the position meant scrolling away from it, and the king highlight faded after a moment. The side to move was also easy to lose, especially while the clock was paused.

The result now stays with the board:

• A banner directly above the squares names the side to move, using the piece-color names (for example “Amethyst to move”). In check it turns gold (“Check · Amethyst to move”). On checkmate it turns red (“Checkmate · Amethyst wins”) and a stamp sits on the board.
• The checked or mated king stays marked, with a + or # badge, instead of flashing and disappearing.
• A colored bar sits on that side’s edge of the board, including after Flip board, so you can see who is to move without reading the clocks.
• The clock for the side to move stays highlighted even when the timer is paused. The other clock dims. Neither is marked once the game is over.
• Stalemate and other draws use the same on-board banner and stamp.

Demo

Phone-width view of the screenshot position (… 16. c5 Qf2#):

Checkmate banner, stamp, and marked king on a phone-width board

checkmate_and_turn_on_board.mp4

Test

• The screenshot position shows “Checkmate · Amethyst wins” above the board, a checkmate stamp on the board, a red bar on Lilac’s edge, and a # on the king, all in the same view as the board on a phone-sized screen.
• 1. e4 f6 2. Qh5+ shows a gold check banner, a gold bar on the checked side, and a + on the king.
• The starting position and 1. e4 show the side to move on the banner, the edge bar, and the clock. Flip board moves the bar to the other edge.
• A stalemate shows “Stalemate” on the board. Light and dark themes keep the banner readable.

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[github-actions]

🎉 This PR is included in version 1.9.0 🎉

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