Enables dynamic evaluation of url defaults at runtime

[thaqebon]

This PR refactors the urlDefaults implementation to support dynamic url default values by converting it into a function. This allows defaults to be computed or resolved at runtime, rather than being a fixed object.

Usage

// Passing a function setUrlDefaults(() => usePage().props.urlDefaults); // Still supports passing a plain object setUrlDefaults({ domain: props.initialPage.props.domain });