Copilot/implement drag and drop (#64)

* Implement drag and drop of pieces on the chess board

Add drag-and-drop support alongside the existing two-click piece
movement. Uses tkinter ButtonPress/ButtonRelease bindings on each
board square button to detect when a piece is dragged from one
square to another. The target square is determined via
winfo_containing() and the move is injected into the FreeSimpleGUI
event loop via write_event_value(), where the existing move
validation and execution code handles it.

Agent-Logs-Url: https://github.com/fsmosca/Python-Easy-Chess-GUI/sessions/8a412d60-5a44-4b76-91b9-d11bd4e558c2

Co-authored-by: fsmosca <22366935+fsmosca@users.noreply.github.com>

* Address code review feedback: optimize widget lookup and add debug logging

Agent-Logs-Url: https://github.com/fsmosca/Python-Easy-Chess-GUI/sessions/8a412d60-5a44-4b76-91b9-d11bd4e558c2

Co-authored-by: fsmosca <22366935+fsmosca@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: fsmosca <22366935+fsmosca@users.noreply.github.com>

Author: fsmosca