Hooks, improvements, and hacks to puter.com, and implementation of a puter integration

[dsagal]

grist-static changes:

• Pass Python errors back through pyCall
• Support importing Excel files by supporting binary data and choosing the right parser.
• Various hooks to allow overriding or extending grist-static behavior.
• Hack to add a "save" hook for when "Save document" button is clicked.

puter.com integration, via page/index_puter.html:

• Support opening and saving .grist files.
• Support opening all file types supported for imports (csv, xlsx, etc)
• Support saving csv files back to the original location, with a warning.
• Show logged-in Puter user (not important, but we don't have UI to avoid showing any user)
• Show when the file is saved/unsaved, and prompt to save when needed.

[dsagal]

There are no tests... I manually tested some flows in Puter, but there is a high risk this breaks something with other uses of grist-static, like the csv viewer. Might be safer to release to a separate URL for the sake of Puter, and leave a proper update of grist-static for when someone has time to poke at grist-static a bit more?

[dsagal]

I released for now into s3://grist-static/experiments/ePUTER01/, with the entrypoint at index_puter2.html (the "2" suffix because i couldn't get past caching of a bad version). That's enough to get the Puter grist app working, so I'll actually update it to that (vs the current pointer to Grist SaaS on prod), but happy to update if/when it's available in a more permanent place.