Use x_user_agent cookie for platform detection in Hotwire Native

Turbo's offline/service worker sets an x_user_agent cookie with the
original browser user agent. This ensures platform detection works
correctly for Hotwire Native apps where service worker requests may
not carry the overridden user agent header.

app/controllers/concerns/set_platform.rb

@@ -7,6 +7,6 @@ module SetPlatform
 
   private
     def platform
-      @platform ||= ApplicationPlatform.new(request.user_agent)
+      @platform ||= ApplicationPlatform.new(cookies[:x_user_agent].presence || request.user_agent)
     end
 end