Enable iframe sandbox (#1178)

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:####`

    
<!-- This is an auto-generated description by cubic. -->
---

## 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.

<!-- End of auto-generated description by cubic. -->

src/components/preview_panel/PreviewIframe.tsx

@@ -544,6 +544,7 @@ export const PreviewIframe = ({ loading }: { loading: boolean }) => {
           </div>
         ) : (
           <iframe
+            sandbox="allow-same-origin allow-scripts allow-forms allow-popups allow-modals allow-orientation-lock allow-pointer-lock allow-presentation allow-downloads"
             data-testid="preview-iframe-element"
             onLoad={() => {
               setErrorMessage(undefined);