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/json/codec.go

@@ -0,0 +1,17 @@
+package json
+
+import (
+	"encoding/json"
+)
+
+// Codec implements the encoding.Encoder and encoding.Decoder interfaces for JSON encoding.
+type Codec struct{}
+
+func (Codec) Encode(v map[string]any) ([]byte, error) {
+	// TODO: expose prefix and indent in the Codec as setting?
+	return json.MarshalIndent(v, "", "  ")
+}
+
+func (Codec) Decode(b []byte, v map[string]any) error {
+	return json.Unmarshal(b, &v)
+}