Add React and Radix UI components

[koaning]

Summary

Migrates JavaScript widget codebase from vanilla D3.js to React + Radix UI while maintaining esbuild as the bundler. Both scatter and bar widgets now use React components with Radix UI controls.

Changes

• Added package.json with React, Radix UI, and D3 dependencies
• Updated Makefile with JSX build targets (js-scatter, js-bar, js-all) and minification
• Created js/scatter_widget.jsx with React/Radix UI controls (ToggleGroup for classes, Slider for brush size)
• Created js/bar_widget.jsx with React/Radix UI controls (ToggleGroup for collections)
• Added GitHub Actions CI workflow to verify JS build on push/PR
• Bundle size reduced to ~210KB each (from 1.1MB) thanks to minification

Test plan

• Run make install to set up dependencies
• Run make js-all to build both widgets
• Verify widgets render and function in Jupyter/marimo notebooks