fix(webui): align React and React DOM versions

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Summary

Provide a short overview of the change and why it is needed.

• Aligns react with react-dom at 19.2.6 after the recent React DOM dependency bump.
• Fixes the React runtime version mismatch that prevented the frontend test environment from importing react-dom/client.

Testing

Describe how you validated this change.

• Added or updated automated tests
• Performed manual verification (describe below)
• Not applicable (explain why)

Manual test notes:

npm run build passes.
npx vitest run passes: 13 files, 130 tests.
npm ls react react-dom shows all React packages deduped to 19.2.6.
npm run lint was attempted but fails on existing React Hooks plugin diagnostics unrelated to this dependency alignment.

Checklist

• Linked the related issue or discussed scope with maintainers
• Ran dotnet format and npm run lint (if touching frontend)
• Updated documentation or release notes where it makes sense
• Confirmed there are no sensitive secrets or credentials committed

Additional Notes

The concrete failure before this change was Error: Incompatible React versions with react: 19.2.5 and react-dom: 19.2.6, causing every Vitest suite to fail before tests executed.