fix(navigation): open download queue from snackbar actions

lib/services/shell_navigation_service.dart

@@ -1,5 +1,7 @@
 import 'package:flutter/widgets.dart';
 
+enum ShellTab { home, library, repository, settings }
+
 class ShellNavigationService {
   static final GlobalKey<NavigatorState> homeTabNavigatorKey =
       GlobalKey<NavigatorState>();
@@ -10,6 +12,29 @@ class ShellNavigationService {
 
   static int _currentTabIndex = 0;
   static bool _showRepoTab = false;
+  static Object? _tabSelectionOwner;
+  static ValueChanged<ShellTab>? _tabSelectionHandler;
+
+  static void registerTabSelectionHandler({
+    required Object owner,
+    required ValueChanged<ShellTab> handler,
+  }) {
+    _tabSelectionOwner = owner;
+    _tabSelectionHandler = handler;
+  }
+
+  static void unregisterTabSelectionHandler(Object owner) {
+    if (!identical(_tabSelectionOwner, owner)) return;
+    _tabSelectionOwner = null;
+    _tabSelectionHandler = null;
+  }
+
+  static bool requestTab(ShellTab tab) {
+    final handler = _tabSelectionHandler;
+    if (handler == null) return false;
+    handler(tab);
+    return true;
+  }
 
   static void syncState({
     required int currentTabIndex,