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/config/performance_test.go

@@ -0,0 +1,140 @@
+package config
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPerformanceConfig_Defaults(t *testing.T) {
+	content := `
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	config, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.NoError(t, err)
+
+	// When performance section is missing, defaults should be applied
+	assert.True(t, config.Performance.Enable)
+	assert.Equal(t, 15*time.Second, config.Performance.Every)
+	assert.Equal(t, 1*time.Hour, config.Performance.MaxAge)
+	assert.Equal(t, 5*time.Minute, config.Performance.GC)
+}
+
+func TestPerformanceConfig_CustomValues(t *testing.T) {
+	content := `
+performance:
+  enable: true
+  every: 30s
+  maxAge: 12h
+  gc: 10m
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	config, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.NoError(t, err)
+
+	assert.True(t, config.Performance.Enable)
+	assert.Equal(t, 30*time.Second, config.Performance.Every)
+	assert.Equal(t, 12*time.Hour, config.Performance.MaxAge)
+	assert.Equal(t, 10*time.Minute, config.Performance.GC)
+}
+
+func TestPerformanceConfig_Disabled(t *testing.T) {
+	content := `
+performance:
+  enable: false
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	config, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.NoError(t, err)
+
+	assert.False(t, config.Performance.Enable)
+	// Duration defaults should still apply
+	assert.Equal(t, 15*time.Second, config.Performance.Every)
+	assert.Equal(t, 1*time.Hour, config.Performance.MaxAge)
+	assert.Equal(t, 5*time.Minute, config.Performance.GC)
+}
+
+func TestPerformanceConfig_PartialValues(t *testing.T) {
+	content := `
+performance:
+  every: 10s
+  maxAge: 6h
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	config, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.NoError(t, err)
+
+	// enable should default to true
+	assert.True(t, config.Performance.Enable)
+	assert.Equal(t, 10*time.Second, config.Performance.Every)
+	assert.Equal(t, 6*time.Hour, config.Performance.MaxAge)
+	// gc should use default
+	assert.Equal(t, 5*time.Minute, config.Performance.GC)
+}
+
+func TestPerformanceConfig_InvalidEvery(t *testing.T) {
+	content := `
+performance:
+  every: 500ms
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	_, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "every must be at least 1s")
+}
+
+func TestPerformanceConfig_InvalidMaxAge(t *testing.T) {
+	content := `
+performance:
+  maxAge: 0s
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	_, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "maxAge must be greater than 0")
+}
+
+func TestPerformanceConfig_InvalidGC(t *testing.T) {
+	content := `
+performance:
+  gc: 0s
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	_, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "gc must be greater than 0")
+}
+
+func TestPerformanceConfig_ComplexDurations(t *testing.T) {
+	content := `
+performance:
+  every: 1m30s
+  maxAge: 2h10m
+  gc: 1m
+models:
+  model1:
+    cmd: path/to/cmd --port ${PORT}
+`
+	config, err := LoadConfigFromReader(strings.NewReader(content))
+	assert.NoError(t, err)
+
+	assert.Equal(t, 90*time.Second, config.Performance.Every)
+	assert.Equal(t, (2*time.Hour)+(10*time.Minute), config.Performance.MaxAge)
+	assert.Equal(t, 1*time.Minute, config.Performance.GC)
+}