Org settings: upload and remove the organization logo

[tefkah]

The org avatar endpoint (POST /api/accounts/:slug/avatar) already existed, but nothing in the UI used it. This adds the missing pieces.

• Org settings: owners get a file picker, an "Upload logo" button and a "Remove" button. JPEG, PNG, GIF or WebP up to 5 MB, as the endpoint already enforces.
• New DELETE /api/accounts/:slug/avatar (owners only): clears avatar_url and deletes the stored files.
• Old files are deleted on upload: uploading a new logo removes the previous file under avatars/<orgId>/. The account-delete and org-delete routes now share the same cleanup helper.
• Personal Settings: removed the "Upload avatar" form. It posted to /api/accounts/me/avatar, which matched /:slug/avatar with slug me and always returned "Organization not found". The personal picture comes from KF Auth anyway, so the page now says "Picture managed by your KF Account".

Before merging

Production doesn't set ASSETS_BASE_URL, so logo URLs default to https://assets.underlay.org, which probably doesn't serve S3_PUBLIC_BUCKET publicly. If so, uploads succeed but the images show as broken. One of these needs to happen first:

• set ASSETS_BASE_URL to the bucket's public URL, or
• make that domain public, or
• serve avatars through the app.

Testing

Typecheck, lint, format check and unit tests (177) pass. I didn't try an upload against a running app with a real bucket.

[tefkah]

wasnt possible to upload avatars for orgs (I created https://www.underlay.org/centeroftrialanderror/ today, hence)

also uploading avatars for your own account did nothing, bc it would try to upload to the /me organization

[isTravis]

Awesome. It's Before Merging note isn't accurate - the buckets/domains are as they should be, this can merge as is.