Add SSO login support

- Add [sso] config section with redirect_uri
- Create mcdsl/sso client when SSO is configured
- Add /login (landing page), /sso/redirect, /sso/callback routes
- Add /logout route
- Update login template with SSO landing page variant
- Bump mcdsl to v1.6.0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/ncruces/go-strftime/README.md

@@ -0,0 +1,5 @@
+# `strftime`/`strptime` compatible time formatting and parsing for Go
+
+[![Go Reference](https://pkg.go.dev/badge/image)](https://pkg.go.dev/github.com/ncruces/go-strftime)
+[![Go Report](https://goreportcard.com/badge/github.com/ncruces/go-strftime)](https://goreportcard.com/report/github.com/ncruces/go-strftime)
+[![Go Coverage](https://github.com/ncruces/go-strftime/wiki/coverage.svg)](https://raw.githack.com/wiki/ncruces/go-strftime/coverage.html)