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>

go.mod

@@ -4,7 +4,7 @@ go 1.25.7
 
 require (
 	git.wntrmute.dev/kyle/goutils v1.21.0
-	git.wntrmute.dev/mc/mcdsl v1.4.0
+	git.wntrmute.dev/mc/mcdsl v1.6.0
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/viper v1.21.0