feat(desktop): build artifact explorer around canonical outputs

[Whiteks1]

Goal

Provide a focused Artifact Explorer built around QuantLab's canonical outputs.

Why

Artifacts are central to auditability and reproducibility. The desktop needs a clear way to inspect them without relying on fragmented legacy views.

Scope

• build or harden an artifact explorer for canonical outputs
• include:
  • artifact list
  • metadata panel
  • preview/inspection area
  • relation to current run or session
  • validation state where available
• prefer reuse of useful research_ui pieces only when they solve a central desktop need
• keep artifact navigation aligned with run detail and compare workflows

Data sources

• run artifacts under outputs/runs/<run_id>/
• paper artifacts under outputs/paper_sessions/<session_id>/
• existing canonical files such as:
  • report.json
  • metrics.json
  • config.json
  • metadata.json

Acceptance criteria

• artifacts are readable and explorable as product objects
• the explorer is visibly based on canonical outputs, not ad hoc filenames
• the user can move from Runs/Run Detail into artifacts without losing context

Out of scope

• no new artifact contract
• no broad migration of all research_ui
• no full log viewer overhaul beyond what is needed for artifact inspection