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/github.com/spf13/viper/flags.go

@@ -0,0 +1,57 @@
+package viper
+
+import "github.com/spf13/pflag"
+
+// FlagValueSet is an interface that users can implement
+// to bind a set of flags to viper.
+type FlagValueSet interface {
+	VisitAll(fn func(FlagValue))
+}
+
+// FlagValue is an interface that users can implement
+// to bind different flags to viper.
+type FlagValue interface {
+	HasChanged() bool
+	Name() string
+	ValueString() string
+	ValueType() string
+}
+
+// pflagValueSet is a wrapper around *pflag.ValueSet
+// that implements FlagValueSet.
+type pflagValueSet struct {
+	flags *pflag.FlagSet
+}
+
+// VisitAll iterates over all *pflag.Flag inside the *pflag.FlagSet.
+func (p pflagValueSet) VisitAll(fn func(flag FlagValue)) {
+	p.flags.VisitAll(func(flag *pflag.Flag) {
+		fn(pflagValue{flag})
+	})
+}
+
+// pflagValue is a wrapper around *pflag.flag
+// that implements FlagValue.
+type pflagValue struct {
+	flag *pflag.Flag
+}
+
+// HasChanged returns whether the flag has changes or not.
+func (p pflagValue) HasChanged() bool {
+	return p.flag.Changed
+}
+
+// Name returns the name of the flag.
+func (p pflagValue) Name() string {
+	return p.flag.Name
+}
+
+// ValueString returns the value of the flag as a string.
+func (p pflagValue) ValueString() string {
+	return p.flag.Value.String()
+}
+
+// ValueType returns the type of the flag as a string.
+func (p pflagValue) ValueType() string {
+	return p.flag.Value.Type()
+}