Add custom cave map mode

lib/utils/custom_map_id.dart

@@ -0,0 +1,18 @@
+const int customMapKey6HexLength = 12;
+
+String? normalizeCustomMapId(String? mapId) {
+  if (mapId == null) {
+    return null;
+  }
+
+  final normalized = mapId.trim().toLowerCase();
+  if (normalized.isEmpty) {
+    return null;
+  }
+
+  if (normalized.length <= customMapKey6HexLength) {
+    return normalized;
+  }
+
+  return normalized.substring(0, customMapKey6HexLength);
+}