Issue #922: add WhatsApp Business Cloud API messaging (#922)

Issue #922: add WhatsApp Business Cloud API messaging (#922) Add WhatsApp config tab, a messaging page with customer conversation history, send-bill-via-WhatsApp buttons on invoices/receipts/quotes/ work orders, and a public webhook for inbound replies and status updates. Includes Whatsapp_lib (Cloud API text + PDF document send), Whatsapp controller + views, Whatsapp_webhook, Whatsapp_message model, a whatsapp_messages table, module/permission/grant seeding, and language keys.