Add color-to-play dot indicator on the board

[Copilot]

Displays a small colored dot in the board's padding area to indicate whose turn it is — red dot on red's side, black dot on black's side. Hidden when the game is finished or when browsing move history. Correctly repositions on board flip.

BoardGui (board-gui.js + board.css)

• New .color-to-play-dot CSS class: absolutely positioned circle, 3% width, white box-shadow for contrast against the brown board background
• New private fields #colorToPlayDot (DOM ref) and #currentColorToPlay (persisted state)
• Dot element created at the end of #drawBoard() (non-mini boards only), ensuring it survives flip() and updateMoveFormat() redraws
• set colorToPlay(color) — public setter; null hides the dot
• get colorToPlayOnBoard() — returns this.#board.getColorToPlay() without a full board copy
• Positioning logic: (color === Color.RED) === this.#flippedRed determines bottom vs. top

PvP (player-vs-player-page.js)

• #updateBoardTurnToPlay() now also sets colorToPlay to colorToPlayOnBoard when in progress, null otherwise
• #handleNavigationEvent() explicitly sets colorToPlay = null when browsing history

PvB (player-vs-bot-page.js)

• New #updateColorToPlayDot() helper using isStatusInProgress()
• Called after FEN load, after player move (shows bot's turn dot while server responds), and inside registerOpponentMove's animation callback (so board state is current)
• Explicitly set to null on resign, cancel, and game-end callbacks

[benckx]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in commit 62606a9. The three conflicting files were:

• board.css — kept both .color-to-play-dot and the new .touch-drag-ghost classes
• board-gui.js — kept both sets of new private fields (#colorToPlayDot/#currentColorToPlay and the touch-drag fields from master)
• changelog.html — kept both changelog entries as separate <li> items