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/logger.go

@@ -0,0 +1,31 @@
+package viper
+
+import (
+	"context"
+	"log/slog"
+)
+
+// WithLogger sets a custom logger.
+func WithLogger(l *slog.Logger) Option {
+	return optionFunc(func(v *Viper) {
+		v.logger = l
+	})
+}
+
+type discardHandler struct{}
+
+func (n *discardHandler) Enabled(_ context.Context, _ slog.Level) bool {
+	return false
+}
+
+func (n *discardHandler) Handle(_ context.Context, _ slog.Record) error {
+	return nil
+}
+
+func (n *discardHandler) WithAttrs(_ []slog.Attr) slog.Handler {
+	return n
+}
+
+func (n *discardHandler) WithGroup(_ string) slog.Handler {
+	return n
+}