refactor(desktop): modularize desktop main process

[Whiteks1]

Summary

• Split desktop/main.js into responsibility-based modules for config, local stores, workspace state, research_ui lifecycle, Stepbit chat, smoke support, window creation, and IPC registration.
• Keep desktop runtime behavior unchanged while reducing main.js to app lifecycle wiring and service composition.

Issue

• Closes desktop(architecture): modularize desktop main process after TypeScript base #353.

Why

• The TypeScript base is already in place, so the next structural boundary is the Electron main process itself.
• This makes the upcoming typed preload work and later renderer migration safer by removing the current monolithic main-process coupling.

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
• Get-ChildItem desktop/main/*.js | ForEach-Object { node --check $_.FullName }
• 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 is intended; this is a main-process structure slice only.
• Main risk is lifecycle drift across extracted services, contained here by keeping the current JS runtime and validating both smoke modes.

Notes

• This PR does not introduce React, Vite, preload bridge changes, or renderer work.
• smoke:fallback failed once locally with a transient Unexpected end of JSON input, then passed on rerun; the final validated run is green.

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