chore: initialize React upgrade with Node version pinning (.nvmrc) [WIP]

[bosun-ai]

Summary

Initialize React upgrade effort by adding an .nvmrc to pin Node.js to v12 for local development consistency. No React-related code changes have been made yet; this PR is a placeholder to track the upgrade work.

Scope of changes

• Add .nvmrc with "12" to align local Node.js version during the upgrade process.

Motivation

• Ensure consistent Node.js environment across contributors while we prepare and validate the React upgrade from 3.1.15 to 17.0.2.

Impact/Risks

• No application code changes in this PR.
• Minimal risk; only affects local development environment when using nvm.

Follow-ups / Next steps

• Update package.json and lockfile to bump React and react-dom to 17.0.2.
• Audit dependencies for React 17 compatibility (e.g., testing libraries, routing, state management).
• Replace deprecated lifecycle methods and patterns if present.
• Verify build tooling (webpack/babel) supports React 17 and update presets/plugins as needed.
• Run and fix unit/integration tests and linting for React 17.
• Perform manual QA in key user flows.

Reviewer call-outs

• Confirm Node 12 is acceptable for development during this phase or propose a higher LTS if our tooling requires it.
• Share any known incompatibilities in our codebase with React 17 to plan changes.

Links

• React 17 release notes: https://reactjs.org/blog/2020/10/20/react-v17.html

This is a work-in-progress PR and should not be merged yet.

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