Add root-level pyproject.toml with dependencies

[bittner]

This is an initial preparation for making Lucas Chess installable and runnable as a Python package from PyPI.

• Lucas Chess can now be run like Python software via uv run bin/LucasR.py.
• Python dependencies are installed automatically, venv activation is handled by uv.
• The setup assumes that various dependencies (e.g. Qt libraries) are already present on the executing computer.

Verified to be working on a recent version of NixOS (25.11, Xantusia).

Related

• Install Lucas Chess from PyPI (aka pip install lucas-chess) #203