Add pyproject packaging and GUI CLI entrypoint

[ProhibitedTV]

Motivation

• Provide a modern packaging layout so the project can be installed with editable installs and expose a CLI entrypoint.
• Define runtime dependencies required by the GUI (PySide6) and the language runtime (choomlang).
• Place the GUI package under apps/gui so source can be discovered by setuptools without moving code.

Description

• Add a root pyproject.toml using setuptools.build_meta with project metadata (name, version, description, requires-python).
• Declare runtime dependencies including PySide6 and choomlang, and register a console script imagechoom = imagechoom_gui.cli:main under [project.scripts].
• Configure setuptools package discovery to map the top-level package directory to apps/gui and find imagechoom_gui* packages.
• Add minimal package files apps/gui/imagechoom_gui/__init__.py (defines __version__) and apps/gui/imagechoom_gui/cli.py (implements main() that creates and runs a Qt QApplication with a simple QLabel).

Testing

• python -m pip install -e . --no-deps attempted but failed due to environment network/proxy restrictions when pip tried to fetch build dependencies (unable to obtain setuptools>=68).
• python -m pip install -e . --no-deps --no-build-isolation succeeded and produced an editable wheel for imagechoom.
• command -v imagechoom succeeded and the console script entry point was registered as shown by importlib.metadata (entry point imagechoom = imagechoom_gui.cli:main).
• python -c "import imagechoom_gui; print(imagechoom_gui.__version__)" succeeded, while python -c "import imagechoom_gui, imagechoom_gui.cli" failed in this environment because PySide6 is not installed (runtime dependency not present in the test environment).

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Codex Task