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/spf13/cobra/MAINTAINERS

@@ -0,0 +1,13 @@
+maintainers:
+- spf13
+- johnSchnake
+- jpmcb
+- marckhouzam
+inactive:
+- anthonyfok
+- bep
+- bogem
+- broady
+- eparis
+- jharshman
+- wfernandes