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

proxy/helpers_test.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/mostlygeek/llama-swap/internal/logmon"
 	"github.com/mostlygeek/llama-swap/proxy/config"
 	"github.com/stretchr/testify/require"
 	"github.com/tidwall/gjson"
@@ -24,7 +25,7 @@ import (
 var (
 	nextTestPort        int = 12000
 	portMutex           sync.Mutex
-	testLogger          = NewLogMonitorWriter(os.Stdout)
+	testLogger          = logmon.NewWriter(os.Stdout)
 	simpleResponderPath = getSimpleResponderPath()
 )
 
@@ -40,13 +41,13 @@ func TestMain(m *testing.M) {
 
 	switch os.Getenv("LOG_LEVEL") {
 	case "debug":
-		testLogger.SetLogLevel(LevelDebug)
+		testLogger.SetLogLevel(logmon.LevelDebug)
 	case "warn":
-		testLogger.SetLogLevel(LevelWarn)
+		testLogger.SetLogLevel(logmon.LevelWarn)
 	case "info":
-		testLogger.SetLogLevel(LevelInfo)
+		testLogger.SetLogLevel(logmon.LevelInfo)
 	default:
-		testLogger.SetLogLevel(LevelWarn)
+		testLogger.SetLogLevel(logmon.LevelWarn)
 	}
 
 	m.Run()