Fix tvOS focus trap on empty Home after fresh install

Render a focusable empty state on tvOS Home when no sections have content,
with an "Open Sources" button that switches the sidebar selection. Without
a focusable view the tvOS focus engine had no target, leaving the sidebar
unreachable after the initial iCloud alert was dismissed.

Also wire the selectedSidebarItem onChange handler into the tvOS TabView,
which was missing and prevented programmatic sidebar selection.

Yattee/Views/Navigation/UnifiedTabView.swift

@@ -664,6 +664,11 @@ struct UnifiedTabView: View {
         .onChange(of: navigationCoordinator?.pendingNavigation) { _, newValue in
             handlePendingNavigation(newValue)
         }
+        .onChange(of: navigationCoordinator?.selectedSidebarItem) { _, newItem in
+            guard let item = newItem else { return }
+            selection = item
+            navigationCoordinator?.selectedSidebarItem = nil
+        }
     }
 
     /// Applies the configured startup tab on first appearance.