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/package-lock.json

@@ -8,6 +8,7 @@
       "name": "ui-svelte",
       "version": "0.0.0",
       "dependencies": {
+        "chart.js": "4.5.1",
         "highlight.js": "^11.11.1",
         "katex": "^0.16.28",
         "lucide-svelte": "^0.563.0",
@@ -120,6 +121,12 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@kurkle/color": {
+      "version": "0.3.4",
+      "resolved": "https://registry.npmjs.org/@kurkle/color/-/color-0.3.4.tgz",
+      "integrity": "sha512-M5UknZPHRu3DEDWoipU6sE8PdkZ6Z/S+v4dD+Ke8IaNlpdSQah50lz1KtcFBa2vsdOnwbbnxJwVM4wty6udA5w==",
+      "license": "MIT"
+    },
     "node_modules/@napi-rs/wasm-runtime": {
       "version": "1.1.3",
       "resolved": "https://registry.npmjs.org/@napi-rs/wasm-runtime/-/wasm-runtime-1.1.3.tgz",
@@ -1096,6 +1103,18 @@
         "url": "https://github.com/sponsors/wooorm"
       }
     },
+    "node_modules/chart.js": {
+      "version": "4.5.1",
+      "resolved": "https://registry.npmjs.org/chart.js/-/chart.js-4.5.1.tgz",
+      "integrity": "sha512-GIjfiT9dbmHRiYi6Nl2yFCq7kkwdkp1W/lp2J99rX0yo9tgJGn3lKQATztIjb5tVtevcBtIdICNWqlq5+E8/Pw==",
+      "license": "MIT",
+      "dependencies": {
+        "@kurkle/color": "^0.3.0"
+      },
+      "engines": {
+        "pnpm": ">=8"
+      }
+    },
     "node_modules/chokidar": {
       "version": "4.0.3",
       "resolved": "https://registry.npmjs.org/chokidar/-/chokidar-4.0.3.tgz",