Implements #11240.
https://github.com/user-attachments/assets/4d2f8021-3e0f-4d39-95df-bcd72bf7545b
# Important Notes
- Fix a Yjs document corruption bug caused by `DeepReadonly` being replaced by a stub; introduce a `DeepReadonly` implementation without Vue dependency.
- Fix right panel sizing when code editor is open.
- Fix right panel slide-in animation.
- `Ast.Function` renamed to `Ast.FunctionDef`.
# Important Notes
The command to run the gui dev environment has been changed. Invoking the old command will print a message about that.
From now on, use `pnpm dev:gui2` in repository root.