feat(pos): WhatsApp Business API integration — send/receive messages, quotations

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>

pos/static/js/sidebar.js

@@ -24,6 +24,8 @@
             { name: 'Facturación', href: '/pos/invoicing', icon: '<path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><line x1="16" y1="13" x2="8" y2="13"/><line x1="16" y1="17" x2="8" y2="17"/><polyline points="10 9 9 9 8 9"/>' },
             { name: 'Contabilidad', href: '/pos/accounting', icon: '<line x1="12" y1="1" x2="12" y2="23"/><path d="M17 5H9.5a3.5 3.5 0 0 0 0 7h5a3.5 3.5 0 0 1 0 7H6"/>' },
             { name: 'Reportes', href: '/pos/reports', icon: '<line x1="18" y1="20" x2="18" y2="10"/><line x1="12" y1="20" x2="12" y2="4"/><line x1="6" y1="20" x2="6" y2="14"/>' },
+            { name: 'Flotillas', href: '/pos/fleet', icon: '<path d="M1 13h22M1 13l2-6h6l2 6M9 7h6l2 6M15 13l2-6M5 17a2 2 0 1 0 0-4 2 2 0 0 0 0 4zM19 17a2 2 0 1 0 0-4 2 2 0 0 0 0 4z"/>' },
+            { name: 'WhatsApp', href: '/pos/whatsapp', icon: '<path d="M21 11.5a8.38 8.38 0 0 1-.9 3.8 8.5 8.5 0 0 1-7.6 4.7 8.38 8.38 0 0 1-3.8-.9L3 21l1.9-5.7a8.38 8.38 0 0 1-.9-3.8 8.5 8.5 0 0 1 4.7-7.6 8.38 8.38 0 0 1 3.8-.9h.5a8.48 8.48 0 0 1 8 8v.5z"/>' },
         ]},
         { label: 'Sistema', items: [
             { name: 'Configuración', href: '/pos/config', icon: '<circle cx="12" cy="12" r="3"/><path d="M19.07 4.93a10 10 0 0 1 0 14.14M4.93 4.93a10 10 0 0 0 0 14.14"/>' },