Avoid data loss on edits in Squiggle AI

[berekuk]

Right now "Save Model" button in Squiggle AI saves the code from workflow, not from playground.

Also, playground edits don't block navigation.

Things to do:

• show a warning that the original AI code will be saved and edits won't be saved, in "Save Model" modal, and possibly on first edit (in a toast?)
• useExitConfirmation in AI playground