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/PlayerPillButtonView.swift

@@ -10,6 +10,8 @@
 import SwiftUI
 
 struct PlayerPillButtonView: View {
+    @Environment(\.appEnvironment) private var appEnvironment
+
     let configuration: ControlButtonConfiguration
     let isPlaying: Bool
     let hasNext: Bool
@@ -199,7 +201,7 @@ struct PlayerPillButtonView: View {
     private var shareButton: some View {
         Group {
             if let video {
-                ShareLink(item: video.shareURL) {
+                ShareLink(item: appEnvironment?.shareURL(for: video) ?? video.shareURL) {
                     genericButtonLabel
                 }
                 .buttonStyle(.plain)