Refactor: Update QBO connection handling to use QboConnectionService for consistency across services and controllers

app/models/customer.rb

@@ -215,7 +215,7 @@ class Customer < ActiveRecord::Base
   def pull
     begin
       raise Exception unless self.id
-      qbo = Qbo.first
+      qbo = QboConnectionService.current!
       @details = qbo.perform_authenticated_request do |access_token|
         service = Quickbooks::Service::Customer.new(company_id: qbo.realm_id, access_token: access_token)
         service.fetch_by_id(self.id)

app/models/estimate.rb

@@ -60,7 +60,7 @@ class Estimate < ActiveRecord::Base
     log "Pulling details for estimate ##{self.id}..."
     begin
       raise Exception unless self.id
-      qbo = Qbo.first
+      qbo = QboConnectionService.current!
       @details = qbo.perform_authenticated_request do |access_token|
         service = Quickbooks::Service::Estimate.new(company_id: qbo.realm_id, access_token: access_token)
         service(:estimate).fetch_by_id(self.id)

app/models/qbo.rb

@@ -12,24 +12,35 @@ class Qbo < ActiveRecord::Base
     
   include QuickbooksOauth
   include Redmine::I18n
+
+  validate :single_record_only, on: :create
   
   # Updates last sync time stamp
   def self.update_time_stamp
     date = DateTime.now
     log "Updating QBO timestamp to #{date}"
-    qbo = Qbo.first
+    qbo = QboConnectionService.current!
     qbo.last_sync = date
     qbo.save
   end
   
+  # Returns the last sync time formatted for display. If no sync has occurred, returns a default message.
   def self.last_sync
-    format_time(Qbo.first.last_sync)
+    qbo = QboConnectionService.current!
+    return t(:label_qbo_never_synced) unless qbo&.last_sync
+    format_time(qbo.last_sync)
   end
 
   private
 
+  # Logs a message with a QBO-specific prefix for easier identification in the logs.
   def self.log(msg)
     logger.info "[QBO] #{msg}"
   end
 
+  # Validates that only one QBO connection record exists in the database. Adds an error if a record already exists.
+  def single_record_only
+    errors.add(:base, "Only one QBO connection allowed") if Qbo.exists?
+  end
+
 end