[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>

cupertino_gallery/lib/widgets/sheet_page.dart

@@ -13,25 +13,20 @@ class SheetPage extends StatelessWidget {
           onPressed: () {
             Navigator.of(context).push(
               CupertinoSheetRoute<void>(
-                scrollableBuilder:
-                    (BuildContext context, ScrollController controller) {
-                      Widget widgetBuilder(BuildContext context) {
-                        return CupertinoPageScaffold(
-                          navigationBar: CupertinoNavigationBar(
-                            middle: const Text('Sheet'),
-                            trailing: GestureDetector(
-                              child: const Icon(CupertinoIcons.xmark),
-                              onTap: () {
-                                Navigator.of(context).pop();
-                              },
-                            ),
-                          ),
-                          child: const Center(child: Text('This is a sheet')),
-                        );
-                      }
-
-                      return widgetBuilder(context);
-                    },
+                builder: (BuildContext context) {
+                  return CupertinoPageScaffold(
+                    navigationBar: CupertinoNavigationBar(
+                      middle: const Text('Sheet'),
+                      trailing: GestureDetector(
+                        child: const Icon(CupertinoIcons.xmark),
+                        onTap: () {
+                          Navigator.of(context).pop();
+                        },
+                      ),
+                    ),
+                    child: const Center(child: Text('This is a sheet')),
+                  );
+                },
               ),
             );
           },