Feat/btcpay support

[d4rp4t]

This PR adds support for BTCPayServer with Cashu plugin (optionally) installed.

[codecov]

Codecov Report

❌ Patch coverage is 8.07914% with 1115 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.86%. Comparing base (2c34e8b) to head (2a9cc24).

Files with missing lines	Patch %	Lines
...cdreams/numo/core/wallet/impl/CdkWalletProvider.kt	0.68%	292 Missing ⚠️
.../com/electricdreams/numo/PaymentRequestActivity.kt	0.00%	211 Missing ⚠️
...ams/numo/core/payment/impl/BTCPayPaymentService.kt	7.69%	131 Missing and 1 partial ⚠️
...com/electricdreams/numo/ndef/CashuPaymentHelper.kt	0.00%	89 Missing ⚠️
...com/electricdreams/numo/core/wallet/WalletError.kt	0.00%	87 Missing ⚠️
...ms/numo/feature/history/PaymentsHistoryActivity.kt	6.74%	81 Missing and 2 partials ⚠️
...com/electricdreams/numo/core/wallet/WalletTypes.kt	0.00%	65 Missing ⚠️
...eams/numo/core/payment/impl/LocalPaymentService.kt	7.89%	35 Missing ⚠️
...ms/numo/feature/settings/BtcPaySettingsActivity.kt	62.76%	21 Missing and 14 partials ⚠️
...icdreams/numo/ui/adapter/PaymentsHistoryAdapter.kt	0.00%	27 Missing ⚠️
... and 10 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #167      +/-   ##
============================================
- Coverage     20.57%   19.86%   -0.72%     
- Complexity      764      805      +41     
============================================
  Files           146      158      +12     
  Lines         18857    20039    +1182     
  Branches       2325     2530     +205     
============================================
+ Hits           3880     3980     +100     
- Misses        14485    15549    +1064     
- Partials        492      510      +18     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[a1denvalu3]

Report: Payment Bypass via NFC Token Parameter Injection in BTCPay Integration

Vulnerability Description:
In BTCPayPaymentService.kt, the redeemToken function constructs the URL to the BTCNutServer by appending the user-supplied NFC token directly into the query string using a StringBuilder without any URL encoding:

val urlBuilder = StringBuilder("${baseUrl()}/cashu/pay-invoice?token=$token")
if (!paymentId.isNullOrBlank()) {
    urlBuilder.append("&invoiceId=$paymentId")
}
val request = Request.Builder()
    .url(urlBuilder.toString())
    // ...

Because the token variable is populated directly from the parsed NFC NDEF payload, an attacker can append a URL fragment identifier (#) to the end of a valid 1-satoshi Cashu token (e.g., cashuA...#).

When this spoofed token is submitted via NFC, the resulting URL becomes:
${baseUrl()}/cashu/pay-invoice?token=cashuA...#&invoiceId=<MERCHANT_INVOICE_ID>

The HTTP client (OkHttp) and the BTCPay server will treat everything after the # as a URL fragment. This effectively drops the &invoiceId= parameter from the server's perspective, decoupling the token from the merchant's pending invoice.
BTCNutServer will successfully process the valid 1-satoshi token, credit it to the store's general balance, and return a 200 OK HTTP response.
Because redeemToken only checks if the HTTP response is successful (code 200..299), it will return a WalletResult.Success.

In PaymentRequestActivity.kt, upon receiving success from paymentService.redeemToken(), the application immediately assumes the local payment invoice is fully settled and invokes handleLightningPaymentSuccess(). It skips any verification of the actual paid amount against the requested invoice amount. This allows an attacker to completely bypass the payment requirement for an invoice of any amount by presenting a valid NFC token worth only a fraction of a cent (1 satoshi).

Proof of Concept (PoC):
An attacker creates an NDEF message mimicking a Cashu token but appends a URL fragment identifier.

1. Attacker purchases goods worth 100,000 satoshis. Merchant uses Numo to generate a payment request.
2. Attacker prepares an NFC card or emulator with a text payload containing a valid 1-satoshi Cashu token followed by a hash character:
   cashuAeyJ0b2tlbiI6W3sibWludCI6Imh0dHBz...#
3. Attacker taps their NFC device to the merchant's Numo POS.
4. Numo reads the token, passes it to paymentService.redeemToken().
5. The request /cashu/pay-invoice?token=cashuA...#&invoiceId=... is sent.
6. Server processes the 1-satoshi token successfully, completely ignoring the invoiceId parameter hidden in the fragment.
7. Numo receives HTTP 200 OK, assumes the 100,000 satoshi invoice is fully paid, and shows the success screen.

Impact:
Critical. Complete bypass of point-of-sale payment enforcement. An attacker can defraud the merchant of goods or services of any value by paying only 1 satoshi.

Remediation:
Properly URL-encode query parameters when constructing URLs. Using OkHttp's HttpUrl.Builder is highly recommended over string concatenation for safe URL construction:

val url = baseUrl().toHttpUrlOrNull()?.newBuilder()
    ?.addPathSegments("cashu/pay-invoice")
    ?.addQueryParameter("token", token)
    ?.apply {
        if (!paymentId.isNullOrBlank()) {
            addQueryParameter("invoiceId", paymentId)
        }
    }
    ?.build()