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/events.go

@@ -5,7 +5,6 @@ package proxy
 const ProcessStateChangeEventID = 0x01
 const ChatCompletionStatsEventID = 0x02
 const ConfigFileChangedEventID = 0x03
-const LogDataEventID = 0x04
 const ActivityLogEventID = 0x05
 const ModelPreloadedEventID = 0x06
 const InFlightRequestsEventID = 0x07
@@ -43,14 +42,6 @@ func (e ConfigFileChangedEvent) Type() uint32 {
 	return ConfigFileChangedEventID
 }
 
-type LogDataEvent struct {
-	Data []byte
-}
-
-func (e LogDataEvent) Type() uint32 {
-	return LogDataEventID
-}
-
 type ModelPreloadedEvent struct {
 	ModelName string
 	Success   bool