Upgrade to react v19

[nabramow]

Describe briefly what this PR is doing and why

Closes #5381

I was reading about implementing MUI dark mode, and it has some issues with Next.js SSR and styles that Next v15 has solutions for. However, to finish migrating to Next.js v15 we need to upgrade to React v19 first.

Not so many changes needed so decided to just go for it.

Please give clear steps for how the reviewer can best test this PR

Please include any necessary dev environment, .env, etc. adjustments.

• Click around the app and check core functionalities to be sure nothing broke

Web frontend checklist

• [ x ] Formatted my code with yarn format
• [ x ] There are no warnings from yarn lint --fix
• [ x ] There are no console warnings when running the app
• [ x ] Added tests where relevant
• [ x ] All tests pass
• [ x ] Clicked around my changes running locally and it works
• [ x ] Checked Desktop, Mobile and Tablet screen sizes

Other
Untick the following if you'd prefer that maintainers don't push commits/merge your branch.

• A maintainer can push commits to my branch
• A maintainer can merge my PR (you can also merge after approval)

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Project	Deployment	Preview	Updated (UTC)
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[aapeliv]

lgtm

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Upgraded to React v19 and Next.js v15 to support modern features and maintain compatibility with latest web standards

🤖 Bot Debug Information

Model: anthropic.claude-4.5-sonnet
Decision: include
Reasoning: This is a major framework upgrade to React v19 and Next.js v15, which is significant technical infrastructure work. While this is mostly internal, it's a notable upgrade that enables future features (like SSR dark mode support) and keeps the platform on modern, supported versions. Major version upgrades of core frameworks like React typically warrant inclusion in release notes as they represent important technical milestones and may have subtle implications for performance and behavior.