fix: locale-aware decimal input rejected before normalization in invoice amount validation

[Delgado74]

Summary

In lib/screens/9receive_screen.dart, the _confirmRequestAmount() method validates
the raw amount text using double.tryParse directly, which rejects comma-decimal inputs
(e.g., 1,5 or 1 000,50) before _normalizeLocalizedNumber() can process them.
This blocks invoice creation for users in several supported locales (DE, FR, IT, PT, RU, ES).

Steps to Reproduce

1. Set your device locale to one that uses comma as the decimal separator
   (e.g., German, French, Italian, Portuguese, Russian, Spanish).
2. Open the receive screen and tap to create an invoice.
3. Enter an amount like 1,5 or 1 000,50.
4. Tap confirm — the amount is rejected as invalid even though
   _normalizeLocalizedNumber() exists and could handle it.

Expected Behavior

Comma-decimal inputs should be accepted and normalized via _normalizeLocalizedNumber()
before validation.

Suggested Fix

• final amount = double.tryParse(_amountController.text.trim());

• final amount = _normalizeLocalizedNumber(_amountController.text.trim());

References

• PR: feat: receive payments directly without Lightning Address #61
• Review comment: feat: receive payments directly without Lightning Address #61 (comment)