feat(desktop): complete the shell workflow block

[Whiteks1]

Summary

This PR:

• adds the first native decision layer to QuantLab Desktop with Candidates, Shortlist, Baseline, and a local persisted store
• turns the shell into a real research workspace by adding native Run Workspace, Artifacts Explorer, Compare v2, and Paper Ops tabs
• upgrades shell interaction with a searchable command palette, decision-aware workflow actions, and safe local artifact file access

Why

This slice matters because:

• the shell needed to move beyond opening browser surfaces and start supporting the actual lab loop: launch -> inspect -> compare -> decide
• the missing product capability was decision continuity, not more pages; this block makes runs actionable objects with memory instead of passive records
• the shell now better reflects the roadmap already documented for QuantLab Desktop v1

Scope

This PR does not:

• add Stepbit-backed reasoning, automation, or model-driven decisions into the shell chat yet
• replace every browser-backed surface; the shell still reuses research_ui where that remains the lowest-friction implementation
• introduce backend ownership for candidate state; shortlist and baseline remain a local desktop store for now

Validation

Validated with:

• node --check desktop/main.js
• node --check desktop/preload.js
• node --check desktop/renderer/app.js
• python -m pytest test/test_research_ui_server.py

Notes

• This is an integration PR for the immediate desktop shell backlog around #196, #201, #200, #199, #198, and #197.
• Candidate state persists to outputs/desktop/candidates_shortlist.json so the shell can retain shortlist and baseline memory without pushing that ownership into QuantLab backend contracts yet.
• The result is intentionally pragmatic: research_ui still exists, but the shell now owns the central workflow surfaces that matter most for launch, review, comparison, and decision.

Closes #196
Closes #201
Closes #200
Closes #199
Closes #198
Closes #197

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