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/Navigation/AppTabNavigation.swift

@@ -37,7 +37,7 @@ struct AppTabNavigation: View {
                         popularNavigationView
                     }
 
-                    if visibleSections.contains(.trending) {
+                    if FeatureFlags.trendingEnabled && visibleSections.contains(.trending) {
                         trendingNavigationView
                     }