Bump: react v19

[saurabhraghuvanshii]

Description

• Upgraded React runtime versions to 19.2.1
• Fixed the build failure caused by an ajv mismatch by adding Ajv 8 so webpack tooling resolves correctly (updated in package.json and lock-file)
• Fixed SSR safety in by guarding sessionStorage access so server-side build doesn’t hit browser-only APIs.
• Updated the React version reference in docs

This PR fixes

Notes for Reviewers

Signed commits

• Yes, I signed my commits.

[arjunmehta-git]

Thanks for taking on the React v19 upgrade, @saurabhraghuvanshii — this has been on the radar for a while and tackling the AJV mismatch and SSR safety together in one PR is the right scope.

Before this can merge, a few things need attention:

• Issue reference — The "This PR fixes #" line is blank. Is this a standalone upgrade, or does it close a filed issue? Please fill that in.
• Build verification — React 18 → 19 is a major version bump. Even if gatsby develop works in lite mode, we need confirmation that gatsby build (production) completes without warnings or errors. React 19 made breaking changes to certain lifecycle behaviors and the concurrent renderer — any deprecated patterns in our components could surface only at build time.
• SSR guard scope — The sessionStorage guard in Banner/index.js is good. Please confirm there are no other window / localStorage / sessionStorage accesses elsewhere in the codebase that would need the same treatment. A quick grep -r 'sessionStorage\|localStorage\|window\.' src/ (excluding node_modules) would tell you.
• package-lock.json — The lock file shows 845 deletions and 28 additions. Please confirm this diff is clean and doesn't inadvertently downgrade or remove packages unrelated to the React/AJV change.

Good initiative. Let's get the build confirmed and this can move forward.

[saurabhraghuvanshii]

@arjunmehta-git build and verified good to merge