feat: publish to PyPI for easy installation

[corentin-core]

Context

Currently the app requires cloning the repo and installing from source. Publishing to PyPI would make installation a one-liner (pip install budget-forecaster or pipx install budget-forecaster) and demonstrates packaging proficiency.

Proposed solution

• Configure pyproject.toml for PyPI publishing (replace or complement current setup.py)
• Add versioning strategy (e.g., __version__ or git tags)
• Add GitHub Actions workflow for automated publishing on release
• Add [project.scripts] entry point so budget-forecaster is available as a CLI command

Acceptance criteria

• pip install budget-forecaster works from PyPI
• budget-forecaster CLI command launches the TUI
• CI publishes automatically on GitHub release
• README updated with pip install instructions

Related issues

• Related to Go public #9 (go public)