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

internal/perf/monitor_windows.go

@@ -0,0 +1,49 @@
+package perf
+
+import (
+	"context"
+	"time"
+
+	"github.com/mostlygeek/llama-swap/internal/logmon"
+	"github.com/shirou/gopsutil/v4/cpu"
+	"github.com/shirou/gopsutil/v4/mem"
+	"github.com/shirou/gopsutil/v4/net"
+)
+
+func getGpuStats(ctx context.Context, every time.Duration, logger *logmon.Monitor) (chan []GpuStat, error) {
+	return nil, ErrNotImplemented
+}
+
+func readSysStats() (SysStat, error) {
+	cpuPcts, err := cpu.Percent(0, true)
+	if err != nil {
+		return SysStat{}, err
+	}
+
+	vmStat, err := mem.VirtualMemory()
+	if err != nil {
+		return SysStat{}, err
+	}
+
+	const toMB = 1024 * 1024
+
+	netIO := make([]NetIOStat, 0)
+	if ioCounters, err := net.IOCounters(true); err == nil {
+		for _, ioc := range ioCounters {
+			netIO = append(netIO, NetIOStat{
+				Name:      ioc.Name,
+				BytesRecv: ioc.BytesRecv,
+				BytesSent: ioc.BytesSent,
+			})
+		}
+	}
+
+	return SysStat{
+		Timestamp:      time.Now(),
+		CpuUtilPerCore: cpuPcts,
+		MemTotalMB:     int(vmStat.Total / toMB),
+		MemUsedMB:      int(vmStat.Used / toMB),
+		MemFreeMB:      int(vmStat.Free / toMB),
+		NetIO:          netIO,
+	}, nil
+}