desktop(react): make React runtime own migrated workstation surfaces

[Whiteks1]

Summary

Closes #430.

This PR makes the selectable React renderer mount the migrated QuantLab workstation shell instead of the previous placeholder runtime. It keeps the legacy renderer as the default and does not touch smoke, research_ui cutoff, or legacy cleanup.

What changed

• Replaced the React placeholder entry with the migrated workstation shell through desktop/renderer/src/App.tsx.
• Reworked QuantLabContext.jsx into a React-owned provider backed by the preload bridge (window.quantlabDesktop) instead of app-legacy.js globals.
• Added React-owned tab routing for migrated surfaces: Runs, Run Detail, Compare, Candidates, System, Experiments, and Paper Ops.
• Kept Launch and Assistant visible as explicit paused React runtime boundaries without expanding product scope.
• Updated System, Experiments, and Paper Ops adapters so they read from the React-owned context rather than simulated legacy state.

Scope boundary

#430 formalizes React runtime ownership of the migrated workstation surfaces. It does not make React the default renderer, change smoke validation, expand Launch, remove research_ui, or retire the legacy shell.

Validation

• npm ci
• npm run -s typecheck
• npm run -s build
• node --test tests/snapshot-status.test.mjs
• git diff --check
• Bounded React launch check: Electron with QUANTLAB_DESKTOP_RENDERER=react stayed up for 12 seconds without early exit

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