Two `UIApplication.shared.open(_:)` calls keep GutenbergKit from building extension-safe

[jkmassel]

Two places open an external URL through UIApplication.shared.open(_:), and they're the only thing stopping GutenbergKit from building as app-extension-safe code.

Detail

ios/Sources/GutenbergKit/Sources/EditorViewController.swift:1206 sends an external main-frame navigation to the system browser:

if let url = navigationAction.request.url {
    Logger.navigation.debug("Opening external URL in system browser: \(url.absoluteString)")
    await UIApplication.shared.open(url)
}

ios/Sources/GutenbergKit/Sources/Services/LockdownModeMonitor.swift:162 backs the Lockdown Mode sheet's "Learn more" button:

onLearnMore: {
    // Open support article directly to the exclusion section using text fragment
    if let url = URL(string: "https://support.apple.com/en-us/105120#:~:text=…") {
        UIApplication.shared.open(url)
    }
}

How it bites

UIApplication.shared is unavailable in app extensions. Building the GutenbergKit scheme with APPLICATION_EXTENSION_API_ONLY=YES fails on exactly these two lines:

LockdownModeMonitor.swift:162:35: error: 'shared' is unavailable in application extensions for iOS: Use view controller based solutions where appropriate instead.
EditorViewController.swift:1206:33: error: 'shared' is unavailable in application extensions for iOS: Use view controller based solutions where appropriate instead.

With both lines commented out, the same build succeeds, so these are the whole list. While they're here, a host can't embed the editor in an app extension (a share extension, say).

Beyond extensions, library code shouldn't reach for an app-wide global it doesn't own when a view- or scene-scoped API does the same job. #669 removed a third use for the same reason (the upload server's background-task assertion, now a ProcessInfo expiring activity). These two don't share that one's main-thread problem — both already run on the main actor.

Suggested fix

• LockdownModeSheet: open the link with SwiftUI's @Environment(\.openURL) inside the sheet, instead of passing an onLearnMore closure that calls UIApplication.
• EditorViewController: open through the editor's own scene, view.window?.windowScene?.open(url, options: nil).

Both compile under -application-extension. Whether either actually opens anything at runtime inside an extension is a separate question, since extensions generally can't hand a URL to the browser. If we want real extension support, the navigation case probably wants an EditorViewControllerDelegate method so the host decides — hosts may prefer an in-app browser anyway.

Found while reviewing #669. Pre-existing; that PR does not touch these paths.