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/Settings/LayoutNavigationSettingsView.swift

@@ -22,6 +22,7 @@ struct LayoutNavigationSettingsView: View {
                 #else
                 VideoActionsSection(settings: settings)
                 LinkActionSection(settings: settings)
+                SharingSection(settings: settings)
                 ClipboardSection(settings: settings)
                 #endif
                 #if os(iOS)
@@ -143,6 +144,32 @@ private struct LinkActionSection: View {
     }
 }
 
+// MARK: - Sharing Section
+
+#if !os(tvOS)
+private struct SharingSection: View {
+    @Environment(\.appEnvironment) private var appEnvironment
+    @Bindable var settings: SettingsManager
+
+    /// Shown only when a source with a web watch page exists — without one
+    /// the toggle can't change anything.
+    private var hasPrivacyFrontendSource: Bool {
+        appEnvironment?.instancesManager.enabledInstances.contains { $0.watchPageBaseURL != nil } ?? false
+    }
+
+    var body: some View {
+        if hasPrivacyFrontendSource {
+            SettingsFormSection("settings.behavior.sharing.header", footer: "settings.behavior.privacyFrontendSharing.footer") {
+                Toggle(
+                    String(localized: "settings.behavior.privacyFrontendSharing"),
+                    isOn: $settings.privacyFrontendSharingEnabled
+                )
+            }
+        }
+    }
+}
+#endif
+
 // MARK: - Clipboard Section
 
 private struct ClipboardSection: View {