fix: return fees_paid_msat from NWC pay_invoice uniffi binding

[portal-wheatley]

Problem

The portal-app uniffi binding for NWC.pay_invoice was returning only the preimage string, silently discarding the fees_paid field from the NIP-47 response:

// before
pub async fn pay_invoice(&self, invoice: String) -> Result<String, AppError> {
    let response = self.inner.pay_invoice(...).await?;
    Ok(response.preimage) // fees_paid thrown away
}

This meant NwcService.sendPayment in portal-app had no way to surface NWC payment fees — it always returned { preimage } with no feeSats, so the activity detail screen always showed "N/A" for fees on NWC payments.

Root Cause

The internal portal-wallet NwcWallet trait was already updated in #149 to return (preimage, fees_paid_msat) — but that change lives in the portal-rest websocket server path. The portal-app uniffi binding (crates/portal-app/src/nwc.rs) is a separate code path used by the React Native client directly, and was not updated.

Fix

Introduce a PayInvoiceResult record (uniffi does not support tuple return types) containing both preimage and fees_paid_msat, and return it from the uniffi-exported pay_invoice:

#[derive(uniffi::Record)]
pub struct PayInvoiceResult {
    pub preimage: String,
    /// Fees in millisatoshis. Zero if the NWC wallet did not report fees (NIP-47 fees_paid is optional).
    pub fees_paid_msat: u64,
}

fees_paid is Option<u64> in the NIP-47 response, so we default to 0 when absent.

Portal-app side

After this lib version is published, NwcService.sendPayment in portal-app can be updated to:

const result = await this.client.payInvoice(paymentRequest);
return {
  preimage: result.preimage,
  feeSats: Math.floor(result.fees_paid_msat / 1000),
};