Maybe/app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  include Onboardable, Localize, AutoSync, Authentication, Invitable, SelfHostable, StoreLocation, Impersonatable, Breadcrumbable, FeatureGuardable, Notifiable
  include Pagy::Backend

  before_action :detect_os
  before_action :set_default_chat

  private
    def detect_os
      user_agent = request.user_agent
      @os = case user_agent
      when /Windows/i then "windows"
      when /Macintosh/i then "mac"
      when /Linux/i then "linux"
      when /Android/i then "android"
      when /iPhone|iPad/i then "ios"
      else ""
      end
    end

    # By default, we show the user the last chat they interacted with
    def set_default_chat
      @last_viewed_chat = Current.user&.last_viewed_chat
      @chat = @last_viewed_chat
    end
end