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/process_test.go

@@ -11,20 +11,21 @@ import (
 	"testing"
 	"time"
 
+	"github.com/mostlygeek/llama-swap/internal/logmon"
 	"github.com/mostlygeek/llama-swap/proxy/config"
 	"github.com/stretchr/testify/assert"
 )
 
 var (
-	debugLogger = NewLogMonitorWriter(os.Stdout)
+	debugLogger = logmon.NewWriter(os.Stdout)
 )
 
 func init() {
 	// flip to help with debugging tests
 	if false {
-		debugLogger.SetLogLevel(LevelDebug)
+		debugLogger.SetLogLevel(logmon.LevelDebug)
 	} else {
-		debugLogger.SetLogLevel(LevelError)
+		debugLogger.SetLogLevel(logmon.LevelError)
 	}
 }
 
@@ -585,7 +586,7 @@ func TestProcess_CustomTimeouts(t *testing.T) {
 		},
 	}
 
-	debugLogger := NewLogMonitorWriter(io.Discard)
+	debugLogger := logmon.NewWriter(io.Discard)
 	process := NewProcess("test-model", 30, modelConfig, debugLogger, debugLogger)
 
 	// Verify the process was created successfully