website: Convert remote MP4 embeds to local AVIF files

[aidanfnv]

Currently we rely on MP4 embeds for most of the model card videos on the FlashDreams website.

This change swaps out the embeds with AVIF files made by downloading and converting those MP4s, so that if one of those links is changed or is otherwise unreachable, the model card videos will still remain undisturbed on the FlashDreams site.

The SANA-WM model cards are already using AVIF, and these will work the same.

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[greptile-apps]

Greptile Summary

Converts remote MP4 model-card embeds into locally hosted AVIF assets.

• Adds animated AVIF clips under the documentation static-assets directory.
• Updates model pages and the model index to render those local assets as images.
• Preserves existing model-card layouts, overlays, alternative text, and zoom behavior.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
docs/source/models/index.rst	Replaces remote model-card video sources with corresponding local AVIF image directives.
docs/source/models/cosmos_predict2.rst	Converts the teaser and generated samples to local AVIF assets while preserving card and attribution markup.
docs/source/models/hy_worldplay.rst	Converts hero and sample clips to local AVIF images using existing compatible styling and zoom conventions.
docs/source/models/lingbot_world.rst	Replaces remote teaser, generated-sample, and recording videos with matching local AVIF assets.
docs/source/models/omnidreams.rst	Migrates teaser, generated samples, and UI recording media from remote MP4 sources to local AVIF files.
docs/source/models/flashvsr.rst	Replaces both the primary output video and low-resolution inset with corresponding local AVIF images.

_{Reviews (2): Last reviewed commit: "website: Reduce use of raw HTML for AVIF..." | Re-trigger Greptile}

[aidanfnv]

/ok to test f0b3648

[aidanfnv]

/ok to test 4a0d1f1