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/viper/experimental.go

@@ -0,0 +1,8 @@
+package viper
+
+// ExperimentalBindStruct tells Viper to use the new bind struct feature.
+func ExperimentalBindStruct() Option {
+	return optionFunc(func(v *Viper) {
+		v.experimentalBindStruct = true
+	})
+}