Redesign admin workbench frontend

[fengting124]

Summary

• redesign admin navigation into Overview, Detections, Evaluations, Models, and Review
• add evaluation frontend API bindings and an evaluation workbench page
• add detection history, model registry, and review queue pages backed by existing APIs
• preserve the current visual language while improving information architecture and usability
• document the frontend redesign spec, implementation plan, and worklog

Verification

• npm run lint
• npm run build
• local route smoke: /admin, /admin/detections, /admin/evaluations, /admin/models, /admin/review returned 200 from Vite
• Playwright screenshots checked for the new admin pages