feature: Add fallback scheme support (LUD-01)

[Danswar]

Adding support for Fallback scheme as per LNURL documentation. Currently lightning-decoder does not support this kind of urls, this PR aims to fix that for the input and the camera reader.

Please note that this standard is widely used in the wild, like for example: SPAR Supermarkets in Switzerland, it would be nice if you can consider this PR.

https://cointelegraph.com/news/spar-store-zug-switzerland-bitcoin-payments-lightning-network https://youtu.be/Qu5VE2JhF54?si=_3DOgKGa7dpm7ArS

Examples of new supported strings/QR codes:

https://app.dfx.swiss/pl/?lightning=LNURL1DP68GURN8GHJ7CTSDYHXGENC9EEHW6TNWVHHVVF0D3H82UNVWQHHQMZLVFJK2ERYVG6RZCMYX33RVEPEV5YEJ9WT

Other references https://github.com/SamSamskies/lnurlpay/pull/36

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Name	Status	Preview	Comments	Updated (UTC)
lightning-decoder	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 16, 2025 0:55am

[TaprootFreak]

Thank you Daniel This update is really needed! 🙏