fix: Add mounted guard after async network calls in AmountScreen

[Delgado74]

Summary

After awaiting network calls in lib_backup/lib/screens/11amount_screen.dart, the State.mounted flag is not re-checked before calling _showPendingSnackBar, _showSuccessSnackBar, or Navigator.of(context).popUntil. If the user navigates away while a request is in flight, these UI calls run against a disposed widget, which can cause exceptions.
Affected locations

_processLNURLPayment (~lines 291–323): after await _invoiceService.sendPaymentToLNURL — snackbar and navigation calls proceed without a mounted check.
_processLightningAddressPayment (~lines 375–409): same pattern with the Lightning Address payment call.

Suggested fix

Add an if (!mounted) return; guard immediately after each await network call and before any snackbar or navigation call. Example:

final paymentResult = await _invoiceService.sendPaymentToLNURL(...);
if (!mounted) return; // ← add this guard
// then snackbar / Navigator calls

Context

This is a pre-existing issue in the original code, not introduced by PR 

fix: Apply CodeRabbit improvements and localization fixes #61 .
Identified during review of PR
Requested by: @Delgado74