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/dotenv/codec.go

@@ -0,0 +1,61 @@
+package dotenv
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+	"strings"
+
+	"github.com/subosito/gotenv"
+)
+
+const keyDelimiter = "_"
+
+// Codec implements the encoding.Encoder and encoding.Decoder interfaces for encoding data containing environment variables
+// (commonly called as dotenv format).
+type Codec struct{}
+
+func (Codec) Encode(v map[string]any) ([]byte, error) {
+	flattened := map[string]any{}
+
+	flattened = flattenAndMergeMap(flattened, v, "", keyDelimiter)
+
+	keys := make([]string, 0, len(flattened))
+
+	for key := range flattened {
+		keys = append(keys, key)
+	}
+
+	sort.Strings(keys)
+
+	var buf bytes.Buffer
+
+	for _, key := range keys {
+		_, err := buf.WriteString(fmt.Sprintf("%v=%v\n", strings.ToUpper(key), flattened[key]))
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return buf.Bytes(), nil
+}
+
+func (Codec) Decode(b []byte, v map[string]any) error {
+	var buf bytes.Buffer
+
+	_, err := buf.Write(b)
+	if err != nil {
+		return err
+	}
+
+	env, err := gotenv.StrictParse(&buf)
+	if err != nil {
+		return err
+	}
+
+	for key, value := range env {
+		v[key] = value
+	}
+
+	return nil
+}