Add an option to share videos via privacy frontend URLs

When enabled (the default), share links to YouTube videos point to the
active source's web watch page instead of youtube.com. Invidious serves
its frontend at the API URL; Piped uses its configured frontend URL.
Videos the active source can't serve (PeerTube, extracted sources) and
sources without a web frontend keep their original links.

The toggle lives in Settings > Layout & Navigation > Sharing on iOS and
macOS, and is shown only when an enabled source has a web watch page.

Yattee/Views/Components/VideoSwipeActionsModifier.swift

@@ -191,7 +191,7 @@ struct VideoSwipeActionsModifier: ViewModifier {
     private func shareVideo() {
         #if os(iOS)
         let activityVC = UIActivityViewController(
-            activityItems: [video.shareURL],
+            activityItems: [appEnvironment?.shareURL(for: video) ?? video.shareURL],
             applicationActivities: nil
         )
         if let windowScene = UIApplication.shared.connectedScenes.first as? UIWindowScene,
@@ -217,7 +217,7 @@ struct VideoSwipeActionsModifier: ViewModifier {
         #elseif os(macOS)
         let pasteboard = NSPasteboard.general
         pasteboard.clearContents()
-        pasteboard.setString(video.shareURL.absoluteString, forType: .string)
+        pasteboard.setString((appEnvironment?.shareURL(for: video) ?? video.shareURL).absoluteString, forType: .string)
         #endif
     }