fix(frontend): revert eslint to v9 — react-hooks plugin incompatible with v10

eslint-plugin-react-hooks@7.0.1 does not support eslint@10.x yet.
Revert to eslint@9 to fix npm ci resolution failure in CI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: acailic