feat: implement dashboard widgets functionality

Backend:
- Add dashboard API router with widget endpoints
- Create dashboard schemas for widget data
- Add dashboard tests

Frontend:
- Enhance Dashboard page with widget components
- Add dashboard service for API calls
- Create reusable dashboard components

OpenSpec:
- Archive add-dashboard-widgets change
- Add dashboard capability specs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/dashboard/index.ts

@@ -0,0 +1,4 @@
+export { StatisticsCard } from './StatisticsCard'
+export { WorkloadWidget } from './WorkloadWidget'
+export { HealthSummaryWidget } from './HealthSummaryWidget'
+export { QuickActions } from './QuickActions'