Add error handling for Google Maps DOM conflicts with React

The Google Maps API and React both manipulate the DOM, which can cause
conflicts where Google Maps removes nodes that React still has references
to. Add graceful error handling:

1. Remove async flag from Google Maps script - use defer only
2. Add try-catch in marker update useEffect to ignore removeChild errors
3. Add cleanup function to properly tear down markers on unmount
4. Log warnings instead of crashing when DOM conflicts occur

This allows the app to continue functioning even when there are minor
DOM reconciliation issues between Google Maps and React.

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/features/stations/utils/maps-loader.ts

@@ -51,8 +51,7 @@ export function loadGoogleMaps(): Promise<void> {
 
     // Create script tag with callback
     const script = document.createElement('script');
-    script.src = `https://maps.googleapis.com/maps/api/js?key=${apiKey}&loading=async&callback=${callbackName}&libraries=places`;
-    script.async = true;
+    script.src = `https://maps.googleapis.com/maps/api/js?key=${apiKey}&callback=${callbackName}&libraries=places`;
     script.defer = true;
 
     script.onerror = () => {