[upstream #17357] Validate the theme docs page against the appearance key manifest #413

The appearance-key guard added in LibreChat-AI#17357 compares the registry with a manifest (packages/data-provider/specs/themedocs.json), so a contributor can append a key to the manifest without writing the docs row. Validate the librechat.ai theme page against that manifest from the docs repository (fetch the manifest from LibreChat-AI/LibreChat dev and fail when a listed key has no row in theme.mdx).

Finding: LibreChat-AI#17357 (comment)
Docs PR that adds the currently missing rows: LibreChat-AI/docs#853