Modernize molplotly: pyproject.toml, drop JupyterDash

[wjm41]

Summary

• Migrate from setup.py to pyproject.toml for modern Python packaging (PEP 621)
• Replace deprecated JupyterDash with standard Dash, which now natively supports Jupyter
• Expand test coverage with comprehensive tests for add_molecules including edge cases, error handling, and SVG rendering
• Update CI workflow to use the modernized package setup
• Add PLAN.md documenting cleanup and modernization roadmap

Test plan

• Verify pip install . works with the new pyproject.toml
• Run pytest tests/ to confirm all tests pass
• Test add_molecules in a Jupyter notebook to verify Dash integration
• Check that the GitHub Actions CI workflow passes

https://claude.ai/code/session_0117V87rftVNjK2yCo3xZK63