Feature Request: Add support for lntbs (signet) prefix

### Summary
The PayRequestPrefix enum is missing the `lntbs` prefix, which is used for Lightning Network invoices on bitcoin signet. This causes `Bolt11PaymentRequest` to throw a `FormatException` when decoding signet invoices.

### Current behavior

```dart
Throws: FormatException: s50 is not a valid format
final req = Bolt11PaymentRequest('lntbs50u1p5ems8psp5cs8yqjk564udl32...');
```

The lntbs hrp matches lntb via startsWith, but then _processAmount tries to parse s50u as the amount string. The leading s (from lntbs) causes Decimal.parse("s50") to fail.

### Expected behavior

Signet invoices with the`lntbs` prefix should decode successfully.

Locate the PayRequestPrefix enum and the Bolt11PaymentRequest decoding path, then inspect _processAmount where the lntbs suffix is currently parsed. Use the provided signet invoice example to verify that decoding completes without a FormatException.

Written by the indexing model from the issue text.