fix: graceful HTTP shutdown, add ErrorBoundary, fix chart responsive height

- main.go: call server.Shutdown() on SIGTERM/SIGINT to drain in-flight requests
- Add React ErrorBoundary component (catches render crashes, shows retry UI)
- Wrap main content area and settings page with ErrorBoundary
- Fix ChartTabs.tsx: replace runtime window.innerWidth check with Tailwind responsive class (h-[500px] md:h-[600px])
- Restarted backend with latest build (health endpoint now returns proper timestamp)

main.go

@@ -181,6 +181,12 @@ func main() {
 	<-quit
 	logger.Info("📴 Shutdown signal received, closing system...")
 
+	// Gracefully shutdown HTTP server (drain in-flight requests)
+	if err := server.Shutdown(); err != nil {
+		logger.Warnf("⚠️ HTTP server shutdown error: %v", err)
+	}
+	logger.Info("✅ HTTP server stopped")
+
 	// Stop all traders
 	traderManager.StopAll()
 	logger.Info("✅ System shut down safely")