Fix sandbox output panel disappearing when resize handle dragged fully down

[AryanBagade]

Summary

The sandbox output panel ("Type Checking Errors" / "Runtime Output") could be dragged completely out of view when using the resize handle, with no way to recover it without refreshing the page.

Fixes #2211

Test Plan

1. Run the sandbox locally (npm run start in website directory)
2. Drag the resize handle between the editor and output panel all the way down
3. Verify the output panel stops at a minimum height and remains visible
4. Verify you can drag it back up
5. Verify normal resizing still works smoothly