[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/ui/home/widgets/home_title.dart

@@ -4,7 +4,6 @@
 
 import 'package:flutter/material.dart';
 import 'package:google_fonts/google_fonts.dart';
-import 'package:provider/provider.dart';
 
 import '../../auth/logout/view_models/logout_viewmodel.dart';
 import '../../auth/logout/widgets/logout_button.dart';
@@ -13,9 +12,14 @@ import '../../core/themes/dimens.dart';
 import '../view_models/home_viewmodel.dart';
 
 class HomeHeader extends StatelessWidget {
-  const HomeHeader({super.key, required this.viewModel});
+  const HomeHeader({
+    super.key,
+    required this.viewModel,
+    required this.logoutViewModel,
+  });
 
   final HomeViewModel viewModel;
+  final LogoutViewModel logoutViewModel;
 
   @override
   Widget build(BuildContext context) {
@@ -37,12 +41,7 @@ class HomeHeader extends StatelessWidget {
                 height: Dimens.of(context).profilePictureSize,
               ),
             ),
-            LogoutButton(
-              viewModel: LogoutViewModel(
-                authRepository: context.read(),
-                itineraryConfigRepository: context.read(),
-              ),
-            ),
+            LogoutButton(viewModel: logoutViewModel),
           ],
         ),
         const SizedBox(height: Dimens.paddingVertical),