bump: Update version to 1.6.0 with complete PNG receipt system

- Full PNG receipt generation system with robust discount detection
- Material Symbols icons integration
- Comprehensive medical consent and oncology patient support
- Multi-source discount/anticipo detection (discountInfo, descuento, subtotal)
- Mobile-optimized receipt layouts with professional styling
- Complete test coverage for all ticket scenarios

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "ap-pos",
-  "version": "1.5.5",
+  "version": "1.6.0",
   "main": "app.js",
   "scripts": {
     "start": "node server.js"