Allowlist additional video providers in the editor preview's frame-src

[LividCasey]

Summary
The editor preview at app.mintlify.com blocks iframe embeds from third-party video hosts via CSP (frame-src / OpaqueResponseBlocking). The same embeds render correctly on the published site - so authors just can't see their videos while editing, even though everything works in production.

Steps to reproduce

1. In the editor, use the embed command with a video URL from a non-whitelisted host, e.g. https://livid.com/watch/UhHv8SkJ_JAL
2. Preview shows a blocked/empty frame (CSP frame-src violation in console)
3. Publish- the embed renders and plays correctly on the live docs site

Context
My case is Livid (livid.com), a video hosting platform: watch pages are oEmbed-compatible with full Open Graph video metadata and an embeddable HTTPS player, and embeds work in published Mintlify docs today. The gap is preview-only. (Support suggested filing this here- thanks Lexi!)

Request
Either of these should solve it:

• Add livid.com to the editor preview's frame-src allowlist, or
• A general mechanism for embeds that already work in published docs to render in preview (e.g., per-project whitelisted embed domains, or mirroring the production frame-src policy in preview)

The second option would presumably help anyone embedding from hosts beyond the current whitelist. Happy to provide test links or connect engineers if useful.

Note: this is related to but distinct from #684 - that request is about frame-ancestors (embedding Mintlify docs elsewhere); this one is about frame-src in the editor preview (rendering third-party embeds inside Mintlify)