refactor: align dashboard with shared design system

[PeterGuy326]

Summary

• replace the Workbench-style dashboard with four focused Ant Design views: overview, queue detail, dead letters, and diagnostics
• align the visual system with fullstack-ai-infra/design-system Warm Agent Workspace pinned at 9d048faa, via a lightweight semantic adapter compatible with React 19
• add responsive navigation, accessible landmarks and touch targets, dark first-paint protection, Inter assets, and Lucide icons
• document setup and operation, and add a CI design-contract check

Validation

• 73 backend tests: 65 passed, 8 integration-only skipped, 0 failed
• dashboard typecheck and production build
• dashboard and backend production dependency audits: no known vulnerabilities
• design contract and diff checks
• Compose config plus live production smoke on port 3001: health, readiness, 3 seeded queues, proxy isolation
• axe landmark-unique, region, and bypass: 0 violations