P18 phase 2: stacked layout for the IDE (editor part) - #30

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4, actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

src/GlPythonWorkspace.tsx:711:11: Cannot reassign variable after render completes

Needs Pyodide in a browser, so it isn't checked here: the worker (run, REPL, Tests in the browser, input() bridge, gturtle) and the UI. Check these in the embed preview (npm run build:embed && npm run preview:embed).