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/MenuCommands.swift

@@ -39,6 +39,7 @@ struct MenuCommands: Commands {
             Button("Trending") {
                 setTabSelection(.trending)
             }
+            .disabled(!FeatureFlags.trendingEnabled)
             .keyboardShortcut("4")
 
             Button("Search") {