refactor(desktop): establish TypeScript base across desktop

[Whiteks1]

Summary

• Add a real TypeScript base for desktop/ with tsconfig.json, global declarations, and a typecheck script.
• Keep main, preload, and the current renderer in JS while consuming desktop/shared contracts natively.

Issue

• Closes desktop(architecture): establish TypeScript base across main, preload, shared, and renderer #354.

Why

• The desktop migration needed a real TypeScript base before modularizing main or introducing the new renderer shell.
• This makes the shared contract layer consumable natively and reduces risk in the next architecture slices.

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:

• npm run typecheck
• node --check main.js
• node --check preload.js
• node --check renderer/app.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 visible desktop behavior change intended; the slice is limited to TypeScript base wiring and narrow JS typing.
• Main risk is typecheck friction in the existing renderer JS, contained here to minimum narrowing changes.

Notes

• This PR intentionally does not modularize main, introduce React or Vite, or add new shared models beyond the existing contract foundation.
• The branch has been refreshed onto current main, including the already-merged real-path smoke semantics from test(desktop): restore real-path smoke semantics #363; this PR does not change smoke semantics itself.

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