Serve the Zitadel login from the Login V2 container

Zitadel v4 redirects every sign-in to /ui/v2/login, which is served by a
separate container (zitadel-login) that this stack didn't run, so logging
in on a fresh instance ended in a 404.

- Add zitadel-login, with its own service account (login-client) whose
  PAT Zitadel writes to a volume only that container mounts.
- Configure the Login V2 base URL (ZITADEL_LOGIN_BASE_URL): its own port
  in dev, same hostname as Zitadel behind the edge in prod.
- Pin zitadel and zitadel-login to v4.19.1 and add healthchecks.
- Run Zitadel as root, as Zitadel's own compose does, so it can write the
  PATs into root-owned mounts (the Linux bind mount and the new volume).
- Route /ui/v2/login on the auth vhost to the login container and drop
  the no-op grpc_set_header.
- Document the extra steps for an instance created before this.

compose.dev.yml

@@ -15,6 +15,12 @@ services:
     ports:
       - "${ZITADEL_EXTERNAL_PORT}:8080"
 
+  # No edge in dev to put the login under Zitadel's own port, so it gets its
+  # own (ZITADEL_LOGIN_BASE_URL in .env points the browser at it).
+  zitadel-login:
+    ports:
+      - "${ZITADEL_LOGIN_HOST_PORT}:3000"
+
   seaweedfs:
     ports:
       - "9333:9333" # master / admin UI