Add feature flag to disable Trending functionality

Introduces a feature flag to disable the Trending section across the app. When disabled, all trending-related UI elements, navigation links, and settings are hidden.

Changes:
- Add trendingEnabled feature flag to FeatureFlags.swift (currently disabled)
- Hide Trending tab in AppTabNavigation, Sidebar, and TVNavigationView
- Remove Trending option from visible sections settings
- Remove Trending option from startup section picker
- Disable Trending menu command and keyboard shortcut
- Prevent Trending URL navigation in OpenURLHandler
- Hide Trending in FavoriteItemView navigation

Shared/Home/FavoriteItemView.swift

@@ -353,7 +353,7 @@ struct FavoriteItemView: View {
         case .history:
             return false
         case .trending:
-            return visibleSections.contains(.trending)
+            return FeatureFlags.trendingEnabled && visibleSections.contains(.trending)
         case .subscriptions:
             return visibleSections.contains(.subscriptions) && accounts.signedIn
         case .popular: