Fix checking for dependencies on sort

lib/mod_list.dart

@@ -247,6 +247,18 @@ class ModList {
       }
     }
 
+    // Check for missing dependencies
+    for (final modId in activeMods.keys) {
+      final mod = mods[modId]!;
+      for (final depId in mod.dependencies) {
+        if (!mods.containsKey(depId)) {
+          loadOrder.errors.add(
+            "Missing dependency: ${mod.name} requires mod with ID $depId",
+          );
+        }
+      }
+    }
+
     // Reset all marks for topological sort
     for (final mod in mods.values) {
       mod.visited = false;