CMS Pharmacy Billing Pro v5.5 — WhatsApp Invoice Sharing

- Added a WhatsApp button to saved Sales invoices.
- Uses the saved customer phone when available; otherwise asks for a WhatsApp number with country code.
- Opens WhatsApp with an invoice summary including invoice/date, items, batch, expiry, qty, MRP, subtotal, discount, GST and payable total.
- User must review and press Send.
- Important limitation: this is a WhatsApp message summary, not an automatically attached PDF. Sending a PDF file automatically requires a configured WhatsApp Business Platform/API and customer opt-in.
- Existing themes, WordPress connector and backup configuration retained.

Validation: ZIP integrity checked. Live WhatsApp sending and real customer delivery are not tested.
