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/OpenURLHandler.swift

@@ -74,6 +74,7 @@ struct OpenURLHandler {
                 focusMainWindow()
             #endif
         case .trending:
+            guard FeatureFlags.trendingEnabled else { return }
             navigation.hideViewsAboveBrowser()
             navigation.tabSelection = .trending
             #if os(macOS)