Linux USB version of the EpsonPrinter API.

What changed:
- Keeps the incoming JSON models for /api/PosPrinter/Receipt and /api/PosPrinter/SaleOfDay
- Replaces COM/SerialPrinter transport with raw writes to Linux USB printer device
- Uses /dev/usb/lp0 by default from appsettings.json
- Builds a standard receipt when parts of the payload are missing
- Pulls fallback values from appsettings.json under PrintSettings:Defaults

Typical run:
  dotnet restore
  dotnet run

Typical endpoint:
  POST /api/PosPrinter/Receipt

Important setting:
  PrintSettings:DevicePath = /dev/usb/lp0