fix: Memory leak in battery API event listeners

The battery level and charging event listeners were added but never
cleaned up when the component unmounts. This caused memory leaks as
anonymous functions couldn't be removed.

Fix:
- Store handler references in variables
- Properly cleanup event listeners in useEffect return function
- Prevents memory leaks on component unmount/remount cycles

client/src/pages/os.tsx

@@ -230,17 +230,34 @@ export default function AeThexOS() {
   const [batteryInfo, setBatteryInfo] = useState<{ level: number; charging: boolean } | null>(null);
 
   useEffect(() => {
+    let battery: any = null;
+    let levelChangeHandler: (() => void) | null = null;
+    let chargingChangeHandler: (() => void) | null = null;
+
     if ('getBattery' in navigator) {
-      (navigator as any).getBattery().then((battery: any) => {
+      (navigator as any).getBattery().then((bat: any) => {
+        battery = bat;
         setBatteryInfo({ level: Math.round(battery.level * 100), charging: battery.charging });
-        battery.addEventListener('levelchange', () => {
+
+        levelChangeHandler = () => {
           setBatteryInfo(prev => prev ? { ...prev, level: Math.round(battery.level * 100) } : null);
-        });
-        battery.addEventListener('chargingchange', () => {
+        };
+        chargingChangeHandler = () => {
           setBatteryInfo(prev => prev ? { ...prev, charging: battery.charging } : null);
-        });
+        };
+
+        battery.addEventListener('levelchange', levelChangeHandler);
+        battery.addEventListener('chargingchange', chargingChangeHandler);
       });
     }
+
+    // Cleanup: remove battery event listeners to prevent memory leak
+    return () => {
+      if (battery) {
+        if (levelChangeHandler) battery.removeEventListener('levelchange', levelChangeHandler);
+        if (chargingChangeHandler) battery.removeEventListener('chargingchange', chargingChangeHandler);
+      }
+    };
   }, []);
 
   const { data: weatherData, isFetching: weatherFetching } = useQuery({