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_musl_linux_386.go

@@ -0,0 +1,52 @@
+// Copyright 2023 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.
+
+package libc // import "modernc.org/libc"
+
+import (
+	"math/bits"
+	"sync/atomic"
+	"unsafe"
+)
+
+type long = int32
+
+type ulong = uint32
+
+// RawMem represents the biggest byte array the runtime can handle
+type RawMem [1<<31 - 1]byte
+
+func a_crash()
+
+func _a_crash(tls *TLS) {
+	a_crash()
+}
+
+func a_cas(p uintptr, t, s int32) int32
+
+func _a_cas(tls *TLS, p uintptr, test, s int32) int32 {
+	return a_cas(p, test, s)
+}
+
+func _a_store(tls *TLS, p uintptr, v int32) {
+	atomic.StoreInt32((*int32)(unsafe.Pointer(p)), v)
+}
+
+func _a_clz_32(tls *TLS, x uint32) int32 {
+	return int32(bits.LeadingZeros32(x))
+}
+
+func _a_ctz_32(tls *TLS, x uint32) int32 {
+	return X__builtin_ctz(tls, x)
+}
+
+func a_or(p uintptr, v int32)
+
+func _a_or(tls *TLS, p uintptr, v int32) {
+	a_or(p, v)
+}
+
+func _a_swap(tls *TLS, p uintptr, v int32) int32 {
+	return atomic.SwapInt32((*int32)(unsafe.Pointer(p)), v)
+}