UI: LLM health status widget in dashboard

[AbirAbbas]

Ref #316, #319

The backend now exposes GET /api/ui/v1/llm/health which returns circuit breaker state for all configured LLM endpoints. This data isn't surfaced in the UI yet.

What to add:

• Dashboard widget showing LLM backend health (healthy/degraded/down)
• Circuit breaker state indicator (closed/open/half-open)
• Last error message and failure count
• Visual alert when circuit opens

API response shape:

{
  "enabled": true,
  "healthy": false,
  "endpoints": [{
    "name": "litellm",
    "circuit_state": "open",
    "healthy": false,
    "consecutive_failures": 3,
    "last_error": "unhealthy status code: 500",
    "last_success": "2026-03-31T17:17:57Z"
  }]
}

This was a key complaint in #316 — no way to tell if the LLM backend was the problem.