fix(portal): emit valid BCP 47 html lang attribute (#14680)

Locales are stored with underscores (e.g. pt_BR), but the HTML lang
attribute requires hyphens (pt-BR). Convert via a helper in the public
portal layouts.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/helpers/portal_helper.rb

@@ -41,6 +41,10 @@ module PortalHelper
     language_map[locale] || locale
   end
 
+  def html_lang_attribute(locale)
+    locale.to_s.tr('_', '-')
+  end
+
   def theme_query_string(theme)
     theme.present? && theme != 'system' ? "?theme=#{theme}" : ''
   end