Tenant subdomains (hongmingwang.moleculesai.app) proxy to EC2 platform
which has no /cp/auth/* routes. Auth UI lives on app.moleculesai.app.
Added getAuthOrigin() that detects SaaS tenant hosts and redirects to
the app subdomain for login/signup. Non-SaaS hosts (localhost, dev)
fall back to PLATFORM_URL as before.
[Molecule-Platform-Evolvement-Manager]
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>