Add STREAM-compatible Interledger payment support and update clients

[soyuz-dev]

Motivation

• Replace the previous sample/demo payment flow with a STREAM-compatible Interledger sender that posts to a configurable remote wallet endpoint.
• Provide typed request/response models and improved error handling for STREAM payments across Kotlin Multiplatform and the web UI.
• Make the CLI/gradle wrapper executable to allow builds to run locally.

Description

• Add StreamPaymentRequest and StreamPaymentResponse data classes and implement InterledgerClient.sendStreamPayment which posts JSON to a configurable streamSenderEndpoint and throws IllegalStateException on non-success HTTP responses.
• Update the Kotlin/JS platform helper to expose an async wrapper sendStreamPaymentAsync that calls sendStreamPayment and keep getHttpClient() implementation for JS.
• Replace the demo client in the React app with createStreamCompatibleInterledgerClient that posts to VITE_STREAM_SENDER_ENDPOINT and returns the parsed STREAM response, and update the UI to use this client and show STREAM-specific messaging.
• Make gradlew executable (mode changed to 100755) and add necessary imports and JSON content type handling in the shared module.

Testing

• No automated tests were run as part of this change.

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Codex Task