devcontainer.json's forwardPorts is a VS Code feature; the devcontainer CLI
(which devcontainer.sh drives) doesn't implement it. Compose only published
3000, so a container started via `./devcontainer.sh up|rebuild` left the
impeccable live helper on 8400 unreachable from the host — and the failure is
silent: the page loads, the injected script 404s, the element picker just never
appears.
Publish 8400:8400 explicitly. The mapping is fixed rather than dynamic because
the injected script hard-codes http://localhost:8400, which also means only one
container at a time can run live mode.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>