[compass_app] Scope LogoutViewModel to route builder — fix #2604 (#2819)

## Description

**Problem:** `LogoutViewModel` was built inside `HomeHeader.build()`, so
each rebuild could create a new instance and a new `logout` command.
That does not match how other Compass view models are scoped (created in
the GoRouter route `builder` and passed in), and it can affect an
in-flight logout when the home screen rebuilds.

**Change:** Create `LogoutViewModel` once in the `/home` route next to
`HomeViewModel`, pass it through `HomeScreen` → `HomeHeader` →
`LogoutButton`, and remove inline construction in `home_title.dart`.
Update `home_screen_test.dart` to build `LogoutViewModel` with fakes and
remove `Provider` wrappers that only supported `context.read()` in the
header.

**Result:** Logout view model lifetime aligns with the home route; no
visual or copy changes (screenshots not needed).

Fixes https://github.com/flutter/samples/issues/2604

---

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

compass_app/app/lib/routing/router.dart

@@ -11,6 +11,7 @@ import '../ui/activities/view_models/activities_viewmodel.dart';
 import '../ui/activities/widgets/activities_screen.dart';
 import '../ui/auth/login/view_models/login_viewmodel.dart';
 import '../ui/auth/login/widgets/login_screen.dart';
+import '../ui/auth/logout/view_models/logout_viewmodel.dart';
 import '../ui/booking/view_models/booking_viewmodel.dart';
 import '../ui/booking/widgets/booking_screen.dart';
 import '../ui/home/view_models/home_viewmodel.dart';
@@ -46,7 +47,14 @@ GoRouter router(AuthRepository authRepository) => GoRouter(
           bookingRepository: context.read(),
           userRepository: context.read(),
         );
-        return HomeScreen(viewModel: viewModel);
+        final logoutViewModel = LogoutViewModel(
+          authRepository: context.read(),
+          itineraryConfigRepository: context.read(),
+        );
+        return HomeScreen(
+          viewModel: viewModel,
+          logoutViewModel: logoutViewModel,
+        );
       },
       routes: [
         GoRoute(