Accept lightning-prefixed LNURLs in Satoshifier

[BullishNode]

Summary

• Accept an optional, case-insensitive lightning: URI scheme before validating an LNURL.
• Return the canonical bare LNURL so callers do not need to duplicate scheme handling.
• Add a regression test for the reported form: a lowercase lightning: scheme followed by an uppercase LNURL.

Testing

• fvm flutter test test — 42 tests passed.
• The parser is also exercised through the companion mobile integration tests, including prefixed LNURL and BOLT11 inputs.

Context

This targets test/liquid-consolidation-v2, which is the Bull SDK revision line currently used by the mobile develop branch.

Companion mobile PR: SatoshiPortal/bullbitcoin-mobile#2498