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/internal/encoding/yaml/codec.go

@@ -0,0 +1,14 @@
+package yaml
+
+import "go.yaml.in/yaml/v3"
+
+// Codec implements the encoding.Encoder and encoding.Decoder interfaces for YAML encoding.
+type Codec struct{}
+
+func (Codec) Encode(v map[string]any) ([]byte, error) {
+	return yaml.Marshal(v)
+}
+
+func (Codec) Decode(b []byte, v map[string]any) error {
+	return yaml.Unmarshal(b, &v)
+}