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/modernc.org/memory/memory64.go

@@ -0,0 +1,10 @@
+// Copyright 2018 The Memory Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build amd64 || amd64p32 || arm64 || arm64be || mips64 || mips64le || mips64p32 || mips64p32le || ppc64 || ppc64le || sparc64 || riscv64 || loong64
+// +build amd64 amd64p32 arm64 arm64be mips64 mips64le mips64p32 mips64p32le ppc64 ppc64le sparc64 riscv64 loong64
+
+package memory // import "modernc.org/memory"
+
+type rawmem [1<<50 - 1]byte