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/proxymanager_loghandlers.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/gin-gonic/gin"
+	"github.com/mostlygeek/llama-swap/internal/logmon"
 )
 
 func (pm *ProxyManager) sendLogsHandlers(c *gin.Context) {
@@ -89,7 +90,7 @@ func (pm *ProxyManager) streamLogsHandler(c *gin.Context) {
 }
 
 // getLogger searches for the appropriate logger based on the logMonitorId
-func (pm *ProxyManager) getLogger(logMonitorId string) (*LogMonitor, error) {
+func (pm *ProxyManager) getLogger(logMonitorId string) (*logmon.Monitor, error) {
 	switch logMonitorId {
 	case "":
 		// maintain the default