Add support for mailto, sms, and tel schemes.

[Phoenix7351]

Fixes an issue where common non-http(s) schemes (mailto, tel, sms) did nothing when tapped.

Note

Low Risk
Localized navigation-handoff change with tests; behavior is limited to opening external URLs outside the web view.

Overview
External URL schemes (mailto:, tel:, sms:, and any non–web-navigable scheme) are now opened via the OS instead of being dropped or only handled on the main navigation path.

URL gains shouldDelegateToSystem (anything with a scheme that is not http/https/about/data/blob) and the in-webview scheme check is renamed to isWebNavigableScheme. Unsupported navigations still cancel in the web view, but opening is centralized in delegateURLToSystem, which uses UIApplication.shared.open on iOS and NSWorkspace.shared.open on macOS, with a shared denial callback for logging.

New-window navigations (WKUIDelegate) now use the same delegation when the target URL is a system scheme, so links like mailto: opened in a new window are handled like normal link taps. Tests cover scheme classification and that NavigationRequest is nil for mailto: actions.

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[davertay-j]

The new logic is just shuffling around conditionals that already exist at the callsite. The logic change should be isolated solely to the createWebViewWith callback.