feat(Dashboard): display average ping response time in PingServiceCard based on historical data

frontend/src/Dashboard.jsx

@@ -157,6 +157,15 @@ function PingServiceCard({ config, ms, previousMs, history = [], loading, isExpa
               />
             )}
           </div>
+          {history.length > 0 && (() => {
+            const sum = history.reduce((a, b) => a + b, 0);
+            const avg = Math.round(sum / history.length);
+            return (
+              <div className="text-muted small mb-2">
+                Среднее за период: <span className="fw-semibold text-body">{avg} мс</span>
+              </div>
+            );
+          })()}
           {history.length > 0 ? (
             <PingSparklineWrap history={history} color={color} />
           ) : (