Crossfade between top-level tabs instead of iOS slide

Switching between the top-level destinations went through the default
CupertinoPage slide, which is meant for forward pushes. For a peer
tab-switch it slid the incoming screen in over the outgoing one without
animating the old screen away, leaving it visible in the background.

Give the top-level routes a CustomTransitionPage crossfade; keep the
device-detail route on the default slide since it is a genuine drill-in.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/widget/navigation_transition_test.dart

@@ -0,0 +1,45 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/navigation.dart';
+import 'package:go_router/go_router.dart';
+
+void main() {
+  // The app's real [router] is configured so that switching between top-level
+  // destinations crossfades (a [CustomTransitionPage]) instead of using the
+  // default platform slide, which would otherwise leave the outgoing screen
+  // visible behind the incoming one. Drilling into a detail screen keeps the
+  // default slide (a plain [GoRoute.builder]).
+  List<GoRoute> topLevelRoutes() {
+    final shell = router.configuration.routes.single as ShellRoute;
+    return shell.routes.cast<GoRoute>();
+  }
+
+  GoRoute routeByName(String name) =>
+      topLevelRoutes().firstWhere((r) => r.name == name);
+
+  group('navigation transitions', () {
+    test('top-level destinations use a page builder (crossfade)', () {
+      for (final name in ['home', 'devices', 'status', 'settings', 'about']) {
+        final route = routeByName(name);
+        expect(
+          route.pageBuilder,
+          isNotNull,
+          reason: '"$name" should crossfade via a custom page builder',
+        );
+        expect(route.builder, isNull, reason: '"$name" should not also build');
+      }
+    });
+
+    test('the device detail drill-in keeps the default slide builder', () {
+      final devices = routeByName('devices');
+      final detail = devices.routes.single as GoRoute;
+
+      expect(detail.name, 'deviceDetail');
+      expect(
+        detail.builder,
+        isNotNull,
+        reason: 'the detail push should keep the default platform slide',
+      );
+      expect(detail.pageBuilder, isNull);
+    });
+  });
+}