Add full WhatsApp Cloud API integration for Nexus POS:
- Service layer (whatsapp_service.py): send text, templates, quotations,
order confirmations, stock alerts; process incoming webhooks with AI auto-reply
- Blueprint (whatsapp_bp.py): public webhook endpoints for Meta verification +
incoming messages; authenticated endpoints for send, send-quote, conversations
- Conversation UI (whatsapp.html + whatsapp.js): split-panel messenger with
conversation list, chat bubbles, send input, quote sending; both themes
- Migration v1.4: whatsapp_messages table with phone/direction/status indexes
- Config: WHATSAPP_TOKEN, WHATSAPP_PHONE_ID, WHATSAPP_VERIFY_TOKEN env vars
- Sidebar: WhatsApp nav item under Gestion with message-bubble icon
- Ready for Meta Business credentials (infrastructure complete, no API keys needed)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
