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/components/CaptureDialog.svelte

@@ -427,6 +427,14 @@
         <button onclick={() => dialogEl?.close()} class="btn"> Close </button>
       </div>
     </div>
+  {:else}
+    <div class="flex flex-col items-center justify-center p-12">
+      <p class="text-lg text-txtsecondary">Capture not found</p>
+      <p class="text-sm text-txtsecondary mt-1">The capture may have expired or was never recorded.</p>
+      <div class="mt-4">
+        <button onclick={() => dialogEl?.close()} class="btn">Close</button>
+      </div>
+    </div>
   {/if}
 </dialog>