Wire health probes, host stats, and LLM proxy round-trip

Three things from the latest arcadia-app pull:

- health.ts: client for /api/v1/health{,/:service,/detailed,/host}.
  monitoring.tsx now reads real per-subsystem probe state instead of
  synthesizing it from indirect signals (rate limits, sessions, jobs).
- New Host tab on Monitoring with KPI tiles + per-core CPU bars,
  load-avg cards, memory + swap usage, and per-mount disk bars,
  backed by /api/v1/health/host.
- llm-proxy.ts: typed errors (secret_disabled, ip_not_allowed, etc.)
  and a probeProxy() that round-trips a 1-token chat. settings.tsx's
  "Test connection" in proxy mode now exercises the real endpoint
  instead of just confirming the adapter built. Contract doc flipped
  from "not yet implemented" to "implemented".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/LLM_PROXY_CONTRACT.md

@@ -1,6 +1,6 @@
 # LLM Proxy Contract
 
-> **Status: not yet implemented on the backend.** This document is the contract that `lib-llm-providers-ui` expects from arcadia. Implement `POST /api/v1/ai/llm/chat` server-side to make `mode: "proxy"` work in the client.
+> **Status: implemented.** Backend lives in `arcadia-app` at `apps/arcadia_core/lib/arcadia/ai/llm_proxy*` (see commit `75669f1`). This document remains the contract that `lib-llm-providers-ui` and `app/lib/arcadia/llm-proxy.ts` expect from arcadia — keep it in sync if either side changes.
 
 ## Why a proxy?