Feature: Include lightning address in NWC connection string if the wallet has a receive paylink

[Mannohat]

Usecase: Setting up a lightning piggy by scanning a NWC qr with associated lightning address allows for fetching the receive qr without having to enter the lightning address as an additional step in the setup process.

[bitcoin3us]

Concrete display-wallet use case for this feature:

Lightning Piggy is an ESP32 hardware display wallet (MicroPythonOS). It can connect to LNBits via NWC and show balance, transactions, and a receive QR. The QR is meant to be a persistent Lightning Address so anyone can scan and pay — that's the whole "leave it on a shelf, receive sats" UX.

Today the NWC URL from nwcprovider doesn't include lud16=, so the device has no Lightning Address to render. The user has to manually type their LN address into a separate "Optional LN Address" field in the wallet's settings. That's an extra friction step that wouldn't exist if the URL self-described its receive address — which is exactly what LUD-16 inside the NWC URL is for, and what other NWC services (Alby, Coinos, Mutiny) already do.

Including &lud16=<wallet-lnaddress> whenever the LNBits wallet has a Lightning Address (Pay Links extension or built-in LN-Address) would unblock this and make nwcprovider URLs portable to any client that consumes lud16 from NWC.

Looks like #27 implements exactly this — happy to test against an lnbits.lightningpiggy.com instance once it lands.