Video-led LandingHero: no poster/preload/error handling, and getVideoFrameUrl allowlist is stale

Follow-up from review on #1057.

On video-led landing pages the hero `` (LandingHero.astro) has no `poster`, no `preload`, and no error handling, replacing an `` that had `loading="eager"` + `fetchpriority="high"`. `` is not LCP-priority-eligible, so the hero regresses on every video-led page.

Root issue for the poster fallback: `video-thumbnail.ts` only allowlists `engcomfy.com`, but live hub thumbnails are served from `comfy-hub-assets.comfy.org` (verified: the image-to-3d hero video is `https://comfy-hub-assets.comfy.org/uploads/...mp4`), so `getVideoFrameUrl` returns null for all of them and the fallback is dead in production.

Proposed fix (needs care, so tracked separately):
1. Confirm `comfy-hub-assets.comfy.org` supports Cloudflare `cdn-cgi/media/mode=frame`, then widen `CLOUDFLARE_CDN_DOMAIN` (shared util, site-wide blast radius).
2. Add `poster` (from getVideoFrameUrl) + `preload` + the `ThumbnailDisplay.astro` 3s `paused && readyState < 3` fallback to the hero video.
3. Re-check LCP with Lighthouse.

Reported by @MaanilVerma.

Start by reading LandingHero.astro, video-thumbnail.ts, and ThumbnailDisplay.astro to trace the current video, poster, allowlist, and fallback behavior. Confirm whether comfy-hub-assets.comfy.org supports Cloudflare frame URLs, then verify the hero has the proposed poster, preload, and error fallback behavior. Re-check the result with Lighthouse for LCP.

Written by the indexing model from the issue text.