Support an explicit public Nextcloud URL

[vitormattos]

Summary

Add an explicit public Nextcloud URL contract for remote development environments.

Changes

• allow NEXTCLOUD_HOST to override the default <compose-project>.localhost hostname;
• add NEXTCLOUD_PROTOCOL=http|https with https as the default;
• keep proxy routing, Nextcloud trusted domain/overwrite settings, diagnostics and Playwright base URL aligned to the same public URL;
• make dev-worker urls report the configured public URL;
• add optional NEXTCLOUD_PORT so a remote platform can expose one worker on a concrete loopback port without changing the local shared-proxy default;
• document the remote-development use case;
• add worker contract coverage for an explicit public hostname.

The existing local behavior remains unchanged when these variables are not set.

This is intended for consumers such as Codespaces, where a forwarded port is exposed under a platform-provided hostname instead of *.localhost.