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/vite.config.ts

@@ -30,13 +30,11 @@ export default defineConfig({
     // on the public internet for dev?! haha.
     host: "0.0.0.0",
     allowedHosts: true,
-    proxy: {
-      "/api": "http://localhost:8080", // Proxy API calls to Go backend during development
-      "/logs": "http://localhost:8080",
-      "/upstream": "http://localhost:8080",
-      "/unload": "http://localhost:8080",
-      "/v1": "http://localhost:8080",
-      "/sdapi": "http://localhost:8080",
-    },
+    proxy: Object.fromEntries(
+      ["/api", "/logs", "/upstream", "/unload", "/v1", "/sdapi"].map((path) => [
+        path,
+        process.env.LLAMA_SWAP_URL ?? "http://localhost:8080",
+      ]),
+    ),
   },
 });