Add helpers for ios/android app store links in welcome mailer (#33475)

app/helpers/application_helper.rb

@@ -238,6 +238,14 @@ module ApplicationHelper
     I18n.t 'user_mailer.welcome.hashtags_recent_count', people: number_with_delimiter(people), count: people
   end
 
+  def app_store_url_ios
+    'https://apps.apple.com/app/mastodon-for-iphone-and-ipad/id1571998974'
+  end
+
+  def app_store_url_android
+    'https://play.google.com/store/apps/details?id=org.joinmastodon.android'
+  end
+
   private
 
   def storage_host_var