chore(deps): standardize monorepo on react 19.x for next.js 16 compatibility

[TanmayChaurasia24]

Summary

This PR fully standardizes the monorepo's dependency tree onto React 19 (specifically 19.2.4 and @types/react@19.0.0).

We are adopting React 19 universally because Next.js 16 no longer supports React 18. This resolves the fragmented react and react-dom versioning across root resolutions and app workspaces, which was at risk of causing serious hydration mismatches and inconsistent ref forwarding behavior.

Changes:

• Updated react and react-dom to 19.2.4 inside apps/web/package.json and in top-level definitions.
• Bumped @types/react and @types/react-dom to 19.0.0 in root resolutions.
• Documented the React 19 standard in CONTRIBUTING.md to prevent future version drift.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore (Dependency updates, refactoring, or tooling improvements)

Checklist

• Title follows conventional commits: feat(scope): description or chore(deps): description
• Type check passes cleanly (yarn type-check:ci --force)
• Diff is focused (addresses only the React versioning constraint)

Fixes #28634

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[cubic-dev-ai]

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