Handle sub-satoshi amounts in Lightning invoices

[Copilot]

Per BOLT11 spec, invoices with sub-satoshi amounts only populate millisatoshis field, not satoshis. The extension was previously unable to display amounts for these invoices.

Changes

• Updated amount calculation to fallback to millisatoshis / 1000 when satoshis is undefined
• Applied to both ConfirmPayment and ConfirmPaymentAsync screens

const amountSat = invoice.satoshis || Number(invoice.millisatoshis) / 1000 || 0;

This handles:

• Whole satoshi amounts (via satoshis field)
• Sub-satoshi amounts (via millisatoshis field)
• Zero-amount invoices (fallback to 0)

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