It works.

This placeholder is served on port 3000 of the ap3-ui box (35.200.132.166) and reaches you through Caddy at ap3-platform.test.silencelaboratories.com. Full chain is live: browser -> HTTPS/Let's Encrypt -> Caddy -> 127.0.0.1:3000.

Deploy the real UI by running your own npm dev/build server on port 3000 (bind 0.0.0.0). Stop this placeholder first: sudo systemctl stop ap3-dummy.