refactor(desktop): modularize shell renderer

[Whiteks1]

Summary

This PR:

• modularizes the QuantLab Desktop renderer by splitting utility, decision-store, and tab-rendering logic into focused ES modules
• keeps the current shell workflow intact while turning desktop/renderer/app.js into a smaller orchestration layer
• updates the renderer entrypoint to load as a module and documents the new structure in the desktop README

Why

This slice matters because:

• the shell renderer had become too large to keep extending safely, and future workflow work would become harder to review and maintain
• this refactor reduces the amount of product logic concentrated in one file without changing the operator-facing workflow surfaces

Scope

This PR does not:

• add new product features or alter the shell roadmap
• change QuantLab or Stepbit backend contracts

Validation

Validated with:

• node --check desktop/main.js
• node --check desktop/preload.js
• node --check desktop/renderer/app.js
• node --check desktop/renderer/modules/utils.js
• node --check desktop/renderer/modules/decision-store.js
• node --check desktop/renderer/modules/tab-renderers.js
• python -m pytest test/test_research_ui_server.py

Notes

• The renderer entrypoint now loads via <script type="module">.
• The biggest extraction targets in this slice were: formatting and path utilities, candidate/shortlist store helpers, and shell-native tab renderers.
• This PR is intentionally maintainability-first so the next shell feature slices do not keep inflating a single renderer file.

Closes #204

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