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/ring/buffer_bench_test.go

@@ -0,0 +1,44 @@
+package ring
+
+import "testing"
+
+const benchCap = 600 // matches default MaxAge/Every (1min / 100ms)
+
+func BenchmarkBuffer_PushNoWrap(b *testing.B) {
+	for b.Loop() {
+		buf := NewBuffer[int](b.N + 1)
+		for i := range b.N {
+			buf.Push(i)
+		}
+	}
+}
+
+func BenchmarkBuffer_PushWrap(b *testing.B) {
+	buf := NewBuffer[int](benchCap)
+	b.ResetTimer()
+	for i := range b.N {
+		buf.Push(i)
+	}
+}
+
+func BenchmarkBuffer_Slice(b *testing.B) {
+	buf := NewBuffer[int](benchCap)
+	for i := range benchCap {
+		buf.Push(i)
+	}
+	b.ResetTimer()
+	for range b.N {
+		_ = buf.Slice()
+	}
+}
+
+func BenchmarkBuffer_PushAndSlice(b *testing.B) {
+	buf := NewBuffer[int](benchCap)
+	b.ResetTimer()
+	for i := range b.N {
+		buf.Push(i)
+		if i%benchCap == 0 {
+			_ = buf.Slice()
+		}
+	}
+}