fix: strip lightning scheme prefix from scanned QR codes

[jvsena42]

Fixes #764

This PR fixes LNURL-pay QR code scanning failing with "Decoding error" when the QR code contains a LIGHTNING: URI scheme prefix.

Description

Some wallets and services generate LNURL QR codes with a LIGHTNING: URI scheme prefix (per BOLT specs). The bitkit-core decoder doesn't handle these prefixes, causing the scan to fail.

The app already had a removeLightningSchemes() workaround for deep links, but it wasn't applied to the QR scanner path and didn't handle the LIGHTNING: prefix. This PR:

1. Moves scheme stripping into handleScan() so all input paths benefit (QR scanner, paste, manual entry, deep links)
2. Adds lightning: prefix handling with case-insensitive matching

Preview

QA Notes

1. Scan an LNURL-pay QR code that includes the LIGHTNING: prefix (e.g. LIGHTNING:LNURL1DP68GURN...)
   • Expected: decodes correctly and shows the LNURL pay flow
   • Before fix: showed "Decoding error, unable to interpret provided data"
2. Scan a plain LNURL QR code without prefix (e.g. LNURL1DP68GURN...)
   • Expected: still works as before
3. Scan a regular Lightning invoice QR code
   • Expected: still works as before
4. Test deep link with lnurl: scheme prefix
   • Expected: still works as before

[jvsena42]

E2E
https://github.com/synonymdev/bitkit-android/actions/runs/21760309213