Refactor EditorContext and system managers for improved clarity and consistency

src/app/editor/system/command_manager.h

@@ -15,6 +15,9 @@ ImGuiKey MapKeyToImGuiKey(char key);
 
 class CommandManager {
 public:
+  CommandManager() = default;
+  ~CommandManager() = default;
+  
   using Command = std::function<void()>;
 
   struct CommandInfo {

src/app/editor/system/resource_manager.h

@@ -10,8 +10,8 @@ namespace editor {
 // System resource manager.
 class ResourceManager {
  public:
-  ResourceManager();
-  ~ResourceManager();
+  ResourceManager() : count_(0) {}
+  ~ResourceManager() = default;
 
   void Load(const char* path);
   void Unload(const char* path);