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/libc/libc_unix1.go

@@ -0,0 +1,27 @@
+// Copyright 2024 The Libc 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 unix && !illumos && !(linux && (amd64 || arm64 || loong64 || ppc64le || s390x || riscv64 || 386 || arm)) && !openbsd
+
+package libc // import "modernc.org/libc"
+
+import (
+	"golang.org/x/sys/unix"
+
+	"modernc.org/libc/sys/types"
+)
+
+// ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
+func Xrecvmsg(t *TLS, sockfd int32, msg uintptr, flags int32) types.Ssize_t {
+	if __ccgo_strace {
+		trc("t=%v sockfd=%v msg=%v flags=%v, (%v:)", t, sockfd, msg, flags, origin(2))
+	}
+	n, _, err := unix.Syscall(unix.SYS_RECVMSG, uintptr(sockfd), msg, uintptr(flags))
+	if err != 0 {
+		t.setErrno(err)
+		return -1
+	}
+
+	return types.Ssize_t(n)
+}