fix(webui): align React and React DOM versions

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Summary

• Fix a runtime/import crash introduced by bumping react-dom to 19.2.6 while leaving react locked at 19.2.5.
• Align react to 19.2.6 in webui/package.json and webui/package-lock.json so React's exact-version check passes.

Testing

Describe how you validated this change.

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

Manual test notes:

npx vitest run initially failed every React test module with: Incompatible React versions: react 19.2.5 / react-dom 19.2.6.
After the fix: npx vitest run passes: 13 files, 130 tests.
After the fix: npm run build passes.
npm run lint still fails on existing React hook lint findings surfaced by the upgraded plugin; CI currently runs lint with || true.
Backend validation was not run because dotnet is not installed in this environment.

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

Root cause: React 19.2.6's react-dom/client enforces exact package version parity. The recent dependency bump updated react-dom only, so importing React DOM failed before tests or the app could render.