Replace HTTP basic auth with MCIAS session cookies. Adds mcdsl auth,
csrf, and web packages. Chi router for route-level auth middleware.
Short code redirects (GET /{short}) remain public; all management
pages require MCIAS login. Nord dark theme, layout+page template
pattern matching other platform services.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
