Add a new "move on release" board setting

[veloce]

Add ChessboardSettings.moveOnRelease: when enabled, a tap-to-move is committed on pointer release instead of pointer press, and a square target follows the finger so the destination can be adjusted before releasing.

Closes #93

[Copilot]

Pull request overview

This PR adds an optional “move on release” interaction mode to the interactive chessboard, matching Chess.com-style tap-to-move behavior where the move is committed on pointer/touch release and the intended destination can be adjusted by sliding before lifting.

Changes:

• Add ChessboardSettings.moveOnRelease to defer tap-to-move commitment until pointer up.
• Update board gesture handling to show a square target that tracks the pointer while the destination is being chosen.
• Add widget tests covering commit-on-release, sliding destination changes, target follower behavior, and invalid release handling.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/src/board_settings.dart	Introduces moveOnRelease setting with documentation, equality/hash, and copyWith support.
lib/src/widgets/board.dart	Implements deferred tap-to-move flow with a follower target and commits on pointer up when enabled.
test/widgets/board_test.dart	Adds focused widget tests validating the new move-on-release behavior and visuals.
example/lib/main.dart	Adds a demo toggle in the example app to enable/disable move-on-release.
example/lib/atomic_game_page.dart	Minor formatting-only adjustment.
CHANGELOG.md	Documents the new setting in a 10.2.0 entry.

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