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/Sidebar.swift

@@ -101,7 +101,7 @@ struct Sidebar: View {
                     .id("popular")
                 }
 
-                if visibleSections.contains(.trending) {
+                if FeatureFlags.trendingEnabled && visibleSections.contains(.trending) {
                     NavigationLink(destination: LazyView(TrendingView()), tag: TabSelection.trending, selection: $navigation.tabSelection) {
                         Label("Trending", systemImage: "arrow.up.right.circle.fill")
                             .accessibility(label: Text("Trending"))