proxy,ui: add performance monitoring with Prometheus metrics (#743)

Add a comprehensive performance monitoring system that collects CPU, memory, swap, load average, network IO, and GPU stats. Provides both a REST API for the UI and a Prometheus /metrics endpoint.

Backend changes:
- New internal/perf package with configurable interval-based stats collection
- GPU monitoring via LACT (Unix socket) and nvidia-smi fallback on Linux
- Ring buffer (internal/ring) for time-series stat storage
- Prometheus /metrics endpoint with all system and GPU metrics
- Moved LogMonitor to internal/logmon package
- New PerformanceConfig for hot-reloadable monitoring settings
- REST /api/performance endpoint replacing SSE streaming

UI changes:
- New Performance page with real-time charts for CPU, memory, GPU, and network
- Reusable PerformanceChart component
- LLAMA_SWAP_URL environment variable support
- Improved capture dialog display

Other:
- Example Grafana dashboard for Prometheus metrics
- monitor-test standalone binary
- Config schema and example updates

fixes #596

ui-svelte/src/stores/api.ts

@@ -7,6 +7,7 @@ import type {
   APIEventEnvelope,
   ReqRespCapture,
   InFlightStats,
+  PerformanceResponse,
 } from "../lib/types";
 import { connectionState } from "./theme";
 
@@ -204,3 +205,17 @@ export async function getCapture(id: number): Promise<ReqRespCapture | null> {
     return null;
   }
 }
+
+export async function fetchPerformance(after?: string): Promise<PerformanceResponse | null> {
+  try {
+    const url = after ? `/api/performance?after=${encodeURIComponent(after)}` : "/api/performance";
+    const response = await fetch(url);
+    if (!response.ok) {
+      throw new Error(`HTTP error! status: ${response.status}`);
+    }
+    return await response.json();
+  } catch (error) {
+    console.error("Failed to fetch performance data:", error);
+    return null;
+  }
+}