chore: prepare React upgrade to 17.0.2 (WIP)

[bosun-ai]

Work in progress: preparing React upgrade

Summary

• This PR is a placeholder for upgrading React from 3.1.15 to 17.0.2. No application code changes have been introduced yet.
• A .nvmrc file was added to standardize the local Node.js version (12) for development consistency ahead of the upgrade work.

Scope

• No changes to dependencies or source code related to React have been made in this branch.
• This PR exists to coordinate upcoming work and discussion for the React upgrade.

Impact/Risks

• None at this time; no runtime or build changes have been introduced.

Follow-ups / Reviewer call-outs

• Confirm the target Node.js version needed for React 17 across local, CI, and production environments.
• Plan dependency upgrades (react, react-dom, related tooling) and identify breaking changes or migration steps.
• Outline testing strategy (unit, integration, e2e) for the upgrade.

This is a test PR and should not be merged.

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