chore: draft PR to upgrade React to 17.0.2 (work in progress)

[bosun-ai]

Work in progress: preparing the React upgrade PR. No source changes are included yet; this draft documents intent and initial scope.

Summary

• Goal: Upgrade client React from 16.8.6 to 17.0.2.
• Current state: No code changes committed in this branch yet. Only dependency resolution has been touched (yarn.lock differs), pending updates to package.json and application code.

Planned scope (to be completed in follow-up commits)

• Update package.json dependencies: react and react-dom to 17.0.2; verify related peer dependencies (react-router, react-helmet[-async], styled-components, react-error-boundary, etc.).
• Audit and adjust any deprecated lifecycle usage or strict mode warnings; React 17 is largely a stepping-stone release with minimal breaking changes, but event delegation behavior changed.
• Verify custom event handlers and synthetic event reliance, especially in components using portals, modals, and document-level listeners (e.g., modals, outsideClickHandler, scrollManager).
• Run unit and integration tests; ensure cypress flows pass and address any behavior changes.

Risks/Impact

• Event system changes in React 17 may affect listeners attached to document/window; test modals, tooltips, and outside click handlers.
• Third-party packages with React <17 peer deps may need upgrades or resolutions.

Reviewer notes

• This is a draft PR intended to stage the upgrade; do not merge.
• Call out any areas known to rely on legacy event bubbling (global listeners, legacy context if any).

Follow-ups

• Lock and document exact dependency versions and any resolutions.
• Update flow-typed stubs for react from v16 to v17 as needed.
• Confirm SSR path (hyperion) operates correctly with React 17 and react-helmet-async.

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