This allows almost all the iframe sandbox tokens
https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/iframe#sandbox
except notably for allow-top-navigation*

Note: allow-same-origin should be OK because the main window is under
the `file://` origin while the iframe window is served on
`localhost:####`

    
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## Summary by cubic
Enable sandboxing on the preview iframe to isolate preview content while
preserving needed capabilities (allow-same-origin, scripts, forms,
popups, modals, orientation/pointer lock, presentation, downloads).
Top-level navigation remains disallowed; allow-same-origin is safe
because the app is file:// and the iframe runs on localhost.

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