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

[mozammal-hossain]

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 #2604

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Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

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