Inside the Modal code playground

What this post added

Introduced an interactive code playground that allows users to write and run Modal code directly in their browser. This is implemented using Modal Sandboxes for secure, isolated code execution. The backend spawns a `modal.Sandbox` with the user's code mounted, executes it using `modal run`, and streams logs back. The frontend uses CodeMirror and Xterm.js. This enhances the learning journey and provides a quick way to experiment with Modal features.