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

Makefile

@@ -25,15 +25,15 @@ proxy/ui_dist/placeholder.txt:
 
 # use cached test results while developing
 test-dev: proxy/ui_dist/placeholder.txt
-	go test -short ./proxy/...
-	staticcheck ./proxy/... || true
+	go test -short ./proxy/... ./internal/...
+	staticcheck ./proxy/... ./internal/... || true
 
 test: proxy/ui_dist/placeholder.txt
-	go test -short -count=1 ./proxy/...
+	go test -short -count=1 ./proxy/... ./internal/...
 
 # for CI - full test (takes longer)
 test-all: proxy/ui_dist/placeholder.txt
-	go test -race -count=1 ./proxy/...
+	go test -race -count=1 ./proxy/... ./internal/...
 
 ui/node_modules:
 	cd ui-svelte && npm install