added support for simultan downloads in files and talk, support for background downloads, enhanced loading in files with retry

lib/routing/app_routes.dart

@@ -49,6 +49,17 @@ class AppRoutes {
   static final RouteObserver<PageRoute<dynamic>> chatRouteObserver =
       RouteObserver<PageRoute<dynamic>>();
 
+  /// Root navigator key, set on [MaterialApp]. Lets globally-mounted UI (e.g.
+  /// the downloads tray, which lives above the navigator in `MaterialApp.builder`
+  /// and is therefore never covered by a pushed route) open full-page routes.
+  static final GlobalKey<NavigatorState> rootNavigatorKey =
+      GlobalKey<NavigatorState>();
+
+  /// A context that is a descendant of the root navigator (its overlay), safe to
+  /// pass to `pushScreen`/`showModalBottomSheet` from outside the route tree.
+  static BuildContext? get overlayContext =>
+      rootNavigatorKey.currentState?.overlay?.context;
+
   static void openFolder(BuildContext context, List<String> path) {
     pushScreen(context, withNavBar: false, screen: Files(path: path));
   }