refactor(desktop): establish shared contract foundation

[Whiteks1]

Summary

• Establish the first shared Desktop contract layer under desktop/shared.
• Type the existing workspace, runtime or snapshot, and smoke boundaries via narrow JSDoc without changing runtime behavior.

Issue

• N/A. This foundation slice predates the migration issue block and does not close a tracked GitHub issue.

Why

• The desktop needed a first real shared contract boundary before introducing the larger TypeScript base slice.
• This reduces drift between main, preload, renderer, and smoke without opening renderer or architecture work early.

Scope

This PR does not:

• Add unrelated feature work.
• Expand scope beyond the slice.
• Introduce a second source of truth for the same content or behavior.

Validation

Validated with:

• node --check main.js
• node --check preload.js
• node --check renderer/app.js
• node --check scripts/smoke.js
• npm run smoke:fallback
• npm run smoke:real-path

Duplication Check

• I searched for existing docs, surfaces, or contracts covering the same idea.
• I removed or replaced stale copies instead of keeping both versions.
• I did not introduce alternate lockfiles, package managers, or build paths.

Compatibility / Risk

• No compatibility change intended; this is a narrow contract-typing slice.
• Risk is limited to JSDoc typing drift in the existing JS files.

Notes

• This PR only aligns contracts and narrow type consumption.
• It prepares the follow-up TypeScript base slice and does not change visible desktop behavior.

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