Generic embeds failing on chromium - switch to blob

[rchlfryn]

Description

Fix embedded scripts (e.g., Donorbox donation form) not loading after client-side navigation on Chromium browsers. Chromium does not re-execute scripts in srcDoc (about:srcdoc) iframes when the iframe is recreated during SPA navigation. Switching to blob URLs forces a real document navigation, ensuring scripts execute fresh every time.

Related Issues

#816

Key Changes

• Replace srcDoc with src={blobUrl} on the GenericEmbed iframe
• Create a unique blob URL per mount via URL.createObjectURL, revoked on unmount

How to test

1. Navigate to a page with a GenericEmbed block containing a script (e.g., Donorbox donate page)
2. Verify the widget loads on first visit
3. Navigate away via a site link, then navigate back
4. Verify the widget loads again on Chrome
5. Verify the widget still works on Firefox and Safari

[github-actions]

Preview deployment: https://embed-donate.preview.avy-fx.org