Use the async version of evaluateJavaScript method.

[Kyome22]

When attempting to build the WebUI with Xcode 26.4, it was discovered that passing Any? as a result within a Task causes a "Sending 'result' risks causing data races" compiler error.

At the time of the initial WebUI implementation (March 2024), the async version of evaluateJavaScript() had the following issue:

Calling a function that returns no value (e.g., console.log(); or window.alert();) was expected to succeed and return nil, but it actually triggered a "Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value" crash.

Because of this, we had been using the closure-based callback version of evaluateJavaScript().

However, this issue was fixed by the WebKit team in December 2024, and it now functions as expected in the latest Xcode.
WebKit/WebKit#37519

For these reasons, we are switching to the async version of evaluateJavaScript().

[Copilot]

Pull request overview

This PR updates WebViewProxy to use the async/await variant of WKWebView.evaluateJavaScript(_:), aligning the WebUI implementation with newer Xcode/WebKit behavior and avoiding the “Sending 'result' risks causing data races” compiler error caused by moving an Any? across a Task boundary.

Changes:

• Replaced the continuation + completion-handler wrapper with a direct try await call to evaluateJavaScript(_:).
• Removed the intermediate Task { @MainActor ... } hop that triggered sendability/data-race diagnostics.

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[antiero]

I think I've just found a duplicate of this issue (#94)

[b1ackturtle]

Good catch, thanks!