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

cmd/monitor-test/main.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/mostlygeek/llama-swap/internal/logmon"
+	"github.com/mostlygeek/llama-swap/internal/perf"
+	"github.com/mostlygeek/llama-swap/proxy/config"
+)
+
+func printSysStat(s perf.SysStat) {
+	cores := make([]string, len(s.CpuUtilPerCore))
+	for i, v := range s.CpuUtilPerCore {
+		cores[i] = fmt.Sprintf("%.1f%%", v)
+	}
+	fmt.Printf("[SYS %s]\n", s.Timestamp.Format("15:04:05"))
+	fmt.Printf("  CPU:  %s\n", strings.Join(cores, "  "))
+	fmt.Printf("  Mem:  %d MB used / %d MB total (%d MB free)\n", s.MemUsedMB, s.MemTotalMB, s.MemFreeMB)
+	fmt.Printf("  Swap: %d MB used / %d MB total\n", s.SwapUsedMB, s.SwapTotalMB)
+	fmt.Printf("  Load: %.2f  %.2f  %.2f  (1m 5m 15m)\n", s.LoadAvg1, s.LoadAvg5, s.LoadAvg15)
+}
+
+func printGpuStats(gpus []perf.GpuStat) {
+	for _, g := range gpus {
+		fmt.Printf("[GPU %d %s]\n", g.ID, g.Name)
+		fmt.Printf("  Util:  GPU %.1f%%  Mem %.1f%%\n", g.GpuUtilPct, g.MemUtilPct)
+		fmt.Printf("  Mem:   %d MB used / %d MB total\n", g.MemUsedMB, g.MemTotalMB)
+		fmt.Printf("  Temp:  %d°C   Fan: %.1f%%   Power: %.1f W\n", g.TempC, g.FanSpeedPct, g.PowerDrawW)
+	}
+}
+
+func main() {
+	stream := flag.Bool("stream", false, "stream stats")
+	interval := flag.Duration("t", time.Second, "polling interval (clamped to 1s–1h)")
+	flag.Parse()
+
+	every := *interval
+	if every < time.Second {
+		every = time.Second
+	} else if every > time.Hour {
+		every = time.Hour
+	}
+
+	l := logmon.New()
+	l.SetLogLevel(logmon.LevelDebug)
+
+	s, err := perf.ReadSysStats()
+	if err != nil && err != perf.ErrNotImplemented {
+		fmt.Println("Sys Error:", err)
+		return
+	}
+	printSysStat(s)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	gpuCh, err := perf.GetGpuStats(ctx, every, l)
+	if err != nil && !errors.Is(err, perf.ErrNotImplemented) && !errors.Is(err, perf.ErrNoGpuTool) {
+		fmt.Println("GPU Init Error:", err)
+		return
+	}
+
+	if gpuCh != nil {
+		select {
+		case g := <-gpuCh:
+			printGpuStats(g)
+		case <-ctx.Done():
+			fmt.Println("GPU: timed out waiting for stats")
+		}
+	}
+
+	if *stream {
+		m, _ := perf.New(config.PerformanceConfig{Enable: true, Every: every}, l)
+		m.Start()
+		defer m.Stop()
+		sysCh, gpuCh, unsub := m.Subscribe()
+		defer unsub()
+		for {
+			select {
+			case s := <-sysCh:
+				printSysStat(s)
+			case g := <-gpuCh:
+				printGpuStats(g)
+			}
+		}
+	}
+}