adding missing const

frontend/lib/modules/landmark_map_marker.dart

@@ -7,7 +7,7 @@ class ThemedMarker extends StatelessWidget {
   final Landmark landmark;
   final int position;
 
-  ThemedMarker({
+  const ThemedMarker({
     super.key,
     required this.landmark,
     required this.position
@@ -24,12 +24,12 @@ class ThemedMarker extends StatelessWidget {
         top: 0,
         right: 0,
         child: Container(
-          padding: EdgeInsets.all(5),
+          padding: const EdgeInsets.all(5),
           decoration: BoxDecoration(
             color: Colors.grey[100],
             shape: BoxShape.circle,
           ),
-          child: Text('$position', style: TextStyle(color: Colors.black, fontSize: 25)),
+          child: Text('$position', style: const TextStyle(color: Colors.black, fontSize: 25)),
         ),
       );
     }
@@ -40,7 +40,7 @@ class ThemedMarker extends StatelessWidget {
         children: [
           Container(
             decoration: BoxDecoration(
-              gradient: landmark.visited ? LinearGradient(colors: [Colors.grey, Colors.white]) : APP_GRADIENT,
+              gradient: landmark.visited ? const LinearGradient(colors: [Colors.grey, Colors.white]) : APP_GRADIENT,
               shape: BoxShape.circle,
               border: Border.all(color: Colors.black, width: 5),
             ),
@@ -54,4 +54,4 @@ class ThemedMarker extends StatelessWidget {
       ),
     );
   }
-}
+}