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/golang.org/x/sys/unix/fdset.go

@@ -0,0 +1,27 @@
+// 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.
+
+//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris || zos
+
+package unix
+
+// Set adds fd to the set fds.
+func (fds *FdSet) Set(fd int) {
+	fds.Bits[fd/NFDBITS] |= (1 << (uintptr(fd) % NFDBITS))
+}
+
+// Clear removes fd from the set fds.
+func (fds *FdSet) Clear(fd int) {
+	fds.Bits[fd/NFDBITS] &^= (1 << (uintptr(fd) % NFDBITS))
+}
+
+// IsSet returns whether fd is in the set fds.
+func (fds *FdSet) IsSet(fd int) bool {
+	return fds.Bits[fd/NFDBITS]&(1<<(uintptr(fd)%NFDBITS)) != 0
+}
+
+// Zero clears the set fds.
+func (fds *FdSet) Zero() {
+	clear(fds.Bits[:])
+}