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/proto/proto_methods.go

@@ -0,0 +1,20 @@
+// 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.
+
+// The protoreflect build tag disables use of fast-path methods.
+//go:build !protoreflect
+// +build !protoreflect
+
+package proto
+
+import (
+	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/runtime/protoiface"
+)
+
+const hasProtoMethods = true
+
+func protoMethods(m protoreflect.Message) *protoiface.Methods {
+	return m.ProtoMethods()
+}