feat: teach Chessboard to support DropMoves

[tom-anders]

dropmoves.webm

[Copilot]

Pull request overview

This pull request adds support for DropMoves to the Chessboard widget, enabling pieces to be dragged from outside the board (e.g., from a piece pocket in Crazyhouse) and dropped onto board squares. The implementation broadens the type system from NormalMove to the more general Move type to accommodate both regular moves and drop moves.

Changes:

• Expanded type signatures from NormalMove to Move throughout the codebase to support both normal moves and drop moves
• Added enableDropMoves setting to ChessboardSettings to enable/disable the feature
• Implemented drag target overlays for all board squares when drop moves are enabled
• Added validation for back-rank pawn drops and empty square requirements

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
lib/src/models.dart	Updated onMove callback and Premovable type definitions to accept Move instead of NormalMove
lib/src/board_settings.dart	Added enableDropMoves boolean setting with default value of false
lib/src/widgets/board.dart	Added 64 DragTarget widgets for drop move acceptance, updated _lastDrop type, and added pattern matching for NormalMove checks
lib/src/widgets/animation.dart	Updated preparePieceAnimations to handle both NormalMove and DropMove in the lastDrop parameter
test/widgets/board_test.dart	Added tests for drop moves and drop premoves, updated test app to support Move type, added unsafe type cast for premove promotion
example/lib/main.dart	Added Crazyhouse demonstration with drag-and-drop piece pockets, duplicated ChessboardSettings creation

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[Copilot]

The documentation comment states "If the move has been made with drag and drop, isDrop will be true." However, this is now ambiguous. The isDrop parameter could mean either:

1. A drag-and-drop interaction (vs tap-to-select)
2. A DropMove (vs NormalMove)

Consider updating the documentation to clarify this distinction, especially since the code at line 420 in board.dart sets isDrop: true specifically for DropMoves (pieces dragged from outside the board), not for all drag-and-drop moves.

Suggested change

	/// If the move has been made with drag and drop, `isDrop` will be true.
	/// The optional [isDrop] flag is `true` when the move is a drop move,
	/// i.e. a piece is placed onto the board from outside (as opposed to a
	/// normal move of a piece already on the board). It does not indicate
	/// whether the user interaction was performed via drag-and-drop or
	/// tap-to-select.

[tom-anders]

@veloce I also thought about this, do we want to rename to viaDragAndDrop maybe?

[veloce]

I think this is not even used by the app... But we should leave it in case other folks use it. In any case it is BC so major version.

[veloce]

IIRC I added that because the old app was using this metadata to send to server (don't remember why).