Add @mediadrop to the registry directory

[vasanth-rb]

Summary

Adds @mediadrop to apps/v4/registry/directory.json.

mediadrop is a small registry of file-upload blocks for React, built on react-mediadrop:

• dropzone — drag-and-drop upload with per-file progress and cancel
• avatar-uploader — circular single-image avatar picker
• multi-file-upload-form — multi-file picker with review + explicit submit
• s3-direct-upload — direct-to-S3 upload via a presigned PUT URL

Registry is served at https://www.mediadrop.dev/r/{name}.json — flat, no nested items, content omitted from the source registry.json's files arrays per the directory requirements.

Checklist

• Registry is public and publicly accessible (no auth)
• registry.json conforms to the registry schema
• Flat structure — registry.json and {name}.json files at /r/ (verified against other directory.json entries using the same /r/{name}.json pattern)
• files array in the source registry.json does not include a content property
• Validated the new entry against apps/v4/scripts/validate-registries.mts's zod schema (name regex, homepage URL, {name} placeholder in url, description present)
• Ran shadcn init + shadcn add against the live URLs in a fresh Next.js app — installs cleanly, tsc --noEmit and next build both pass with 0 errors

Test plan

npx shadcn add https://www.mediadrop.dev/r/dropzone.json \
  https://www.mediadrop.dev/r/avatar-uploader.json \
  https://www.mediadrop.dev/r/multi-file-upload-form.json \
  https://www.mediadrop.dev/r/s3-direct-upload.json

All 4 blocks install correctly with the react-mediadrop dependency resolved.

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