theme="auto" only added the .dark class when theme === "dark", so it always
rendered light regardless of the host. Now "auto" probes the host root
(.dark class / data-theme / color-scheme) and falls back to
prefers-color-scheme, re-resolving on system + host changes via a media-query
listener and a MutationObserver. "dark"/"light" are still honored verbatim;
SSR starts from the explicit value and resolves on mount.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>