fix(webui): prevent duplicate visibility change listeners in health check

Ensure the visibility change event listener for health checks is set up only once
and properly removed on destroy to avoid memory leaks and duplicate handlers.

public/js/data-store.js

@@ -100,10 +100,10 @@ document.addEventListener('alpine:init', () => {
                 clearInterval(this.healthCheckTimer);
             }
 
-            // Setup visibility change listener
+            // Setup visibility change listener (only once)
             if (!this._healthVisibilitySetup) {
                 this._healthVisibilitySetup = true;
-                document.addEventListener('visibilitychange', () => {
+                this._visibilityHandler = () => {
                     if (document.hidden) {
                         // Tab hidden - stop health checks
                         this.stopHealthCheck();
@@ -111,7 +111,8 @@ document.addEventListener('alpine:init', () => {
                         // Tab visible - restart health checks
                         this.startHealthCheck();
                     }
-                });
+                };
+                document.addEventListener('visibilitychange', this._visibilityHandler);
             }
 
             // Perform immediate health check
@@ -271,6 +272,9 @@ document.addEventListener('alpine:init', () => {
 
         destroy() {
             this.stopHealthCheck();
+            if (this._visibilityHandler) {
+                document.removeEventListener('visibilitychange', this._visibilityHandler);
+            }
         }
     });
 });