iamnim self-service UI for SSH key registration

Follow-up to #106 subtask 5. Mycelium now exposes `POST/GET/DELETE /api/orgs/{slug}/ssh-keys` and hetznertreehouse fetches them at provision time, but org admins still have to curl mycelium with the admin api_token. Build the iamnim UI: a per-org settings page where org admins paste public keys, iamnim proxies to mycelium under its own admin token, scoped to the admin's pantheon org membership. Out of scope: rotating/expiring keys, FIDO2 hardware-key flows.

Affected repos: iamnim