Integrate hosted services and desktop health diagnostics

## Objective

Define and implement appropriate background-service and health-diagnostic integration for desktop applications, including UI marshaling guidance, cancellation, offline operation, startup readiness, shutdown, and surfacing failures without blocking or destabilizing the message loop.

## Completion guidance

- Keep the implementation focused on the objective while allowing the assignee to improve the approach based on research.
- Document important decisions, rejected alternatives, dependencies, and follow-up work.
- Add or update tests and documentation appropriate to the work.
- Verify locally and in CI where the task changes code or generated artifacts.

No files, tests, or entry points are identified. Start by mapping how the desktop applications currently handle hosted services, health diagnostics, startup, shutdown, cancellation, offline operation, and UI marshaling. Define the integration boundaries and document decisions, rejected alternatives, dependencies, follow-up work, tests, and documentation needed for completion.

Written by the indexing model from the issue text.