Polished login and dashboard shell with Hive branding

The login page now shows identity-provider buttons when providers are configured, even on public instances. On public instances with no provider it explains the instance is open and how to make it private. The dashboard also gets a dedicated layout with the new Hive honeycomb logo in the header, favicon, and login frame, plus the login page now matches the Tuist-style design with a centered, tighter frame and purple gradient background.