Avatar upload block for the UI Library

[mmaxence]

The library already has current-user-avatar for displaying a user's avatar and dropzone for uploads, but there's no block that ties them together into the common "let a user set their own avatar" flow: pick or drop an image, preview it, upload to a Storage bucket, and update the user's metadata so the new avatar shows everywhere.

It would compose the two existing blocks rather than introduce anything new conceptually: dropzone for the file input, a preview with sensible loading and error states, an upload to a configurable bucket, and a metadata update. It fills a gap between "display an avatar" and "upload a file" that most apps end up hand-rolling.

I'd be glad to implement it, starting with Next.js and React to mirror the existing block structure, and follow up with the other frameworks if it's useful. Before writing code I wanted to check whether this fits the direction you have in mind for the library, and whether there's a preferred convention for the bucket and metadata field so it lines up with the rest.