fix(mobile): deeplinking to asset view while viewer is already open (#19812)

* initial attempt at new guard

* do not resolve the route when replaced

* Update gallery_guard.dart comment

---------

Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>

mobile/lib/routing/gallery_guard.dart

@@ -0,0 +1,31 @@
+import 'package:auto_route/auto_route.dart';
+import 'package:immich_mobile/routing/router.dart';
+
+/// Handles duplicate navigation to this route (primarily for deep linking)
+class GalleryGuard extends AutoRouteGuard {
+  const GalleryGuard();
+  @override
+  void onNavigation(NavigationResolver resolver, StackRouter router) async {
+    final newRouteName = resolver.route.name;
+    final currentTopRouteName =
+        router.stack.isNotEmpty ? router.stack.last.name : null;
+
+    if (currentTopRouteName == newRouteName) {
+      // Replace instead of pushing duplicate
+      final args = resolver.route.args as GalleryViewerRouteArgs;
+
+      router.replace(
+        GalleryViewerRoute(
+          renderList: args.renderList,
+          initialIndex: args.initialIndex,
+          heroOffset: args.heroOffset,
+          showStack: args.showStack,
+        ),
+      );
+      // Prevent further navigation since we replaced the route
+      resolver.next(false);
+      return;
+    }
+    resolver.next(true);
+  }
+}