Packaging and PyPI release

[kicka5h]

Ensure the package installs cleanly and the entry point works correctly.

Scope

• `pyproject.toml` with correct metadata, dependencies, and entry point
• Entry point: `snack = "snacks.main:app"`
• Installable via `pipx install snack-stash` and `pip install snack-stash`
• `snack --help` works after install with no import errors

Acceptance criteria

• `pip install -e .` succeeds on Python 3.10, 3.11, 3.12
• `snack --help` shows all commands after install
• Package builds cleanly with `python -m build` (no warnings)
• Published to PyPI and installable from there