Fixes #736

This PR improves Lightning payment failure error messages by parsing the PaymentFailureReason from LDK-node and displaying user-friendly, actionable messages instead of a generic "Your instant payment failed" toast.

Description

Previously, all Lightning payment failures showed the same generic error message regardless of the actual cause. This made it impossible for users to understand why their payment failed or what action to take.

The PR maps PaymentFailureReason enum values to specific user-friendly messages:

Preview

QA Notes

1. Test payment failure messages

1. Open Bitkit with an active Lightning channel
2. Attempt to send a payment to an offline recipient or with an amount that will fail routing
3. Verify the toast message is contextual (not generic)
4. The message should provide actionable guidance

2. Test QuickPay flow

1. Use QuickPay to scan a Lightning invoice
2. If payment fails, verify the error message shown matches the failure type

3. Test background notifications

1. With background notifications enabled, trigger a payment failure
2. Verify the push notification body shows the user-friendly message

4. Regression

• Successful payments should still show success toast
• Payment received notifications should be unaffected