Show placeholder for broken activity images

When an activity image fails to load, display a placeholder with a "broken image" icon instead of the default error widget. This provides better visual feedback to the user.

compass_app/app/lib/ui/activities/widgets/activity_entry.dart

@@ -30,6 +30,7 @@ class ActivityEntry extends StatelessWidget {
           ClipRRect(
             borderRadius: BorderRadius.circular(8),
             child: CachedNetworkImage(
+              errorWidget: (context, url, error) => _ActivityErrorPlaceholder(),
               imageUrl: activity.imageUrl,
               height: 80,
               width: 80,
@@ -66,3 +67,23 @@ class ActivityEntry extends StatelessWidget {
     );
   }
 }
+
+class _ActivityErrorPlaceholder extends StatelessWidget {
+  const _ActivityErrorPlaceholder({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: 80,
+      height: 80,
+      decoration: BoxDecoration(
+        color: Colors.grey[200],
+        borderRadius: BorderRadius.circular(8),
+      ),
+      child: Icon(
+        Icons.broken_image_outlined,
+        color: Theme.of(context).colorScheme.onSurfaceVariant,
+      ),
+    );
+  }
+}