fix: validate locale cookie before setting I18n locale

Fixes I18n::InvalidLocale error when cookie contains invalid/empty locale.

Root cause: ApplicationController#set_locale was using the cookie value
directly without validation, causing failures when users had an empty
or invalid locale stored from previous sessions.

Author: mroderick

app/controllers/application_controller.rb

@@ -133,7 +133,14 @@ def has_access?
 
   def set_locale
     store_locale_to_cookie(params[:locale]) if locale
-    I18n.locale = cookies[:locale] || I18n.default_locale
+    I18n.locale = locale_value
+  end
+
+  def locale_value
+    return I18n.default_locale unless cookies[:locale].present?
+    return I18n.default_locale unless I18n.available_locales.include?(cookies[:locale].to_sym)
+
+    cookies[:locale]
   end
 
   def user_not_authorized