Implement First-Party Mode Setup Success Subtle Notification Component

[hussain-t]

Feature Description

Create a new <FirstPartyModeSetupSuccessSubtleNotification /> component to display a success message upon successfully enabling First-party Mode. This subtle notification should confirm activation, remain visible until dismissed, and be implemented using the new notifications API.

For more details, refer to the Design Doc and the Figma design.

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Acceptance criteria

Implementation Brief

Test Coverage

QA Brief

Changelog entry