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/google.golang.org/protobuf/internal/genid/map_entry.go

@@ -0,0 +1,16 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package genid
+
+import "google.golang.org/protobuf/reflect/protoreflect"
+
+// Generic field names and numbers for synthetic map entry messages.
+const (
+	MapEntry_Key_field_name   protoreflect.Name = "key"
+	MapEntry_Value_field_name protoreflect.Name = "value"
+
+	MapEntry_Key_field_number   protoreflect.FieldNumber = 1
+	MapEntry_Value_field_number protoreflect.FieldNumber = 2
+)