refactor(gui): reorganize background rendering and layout helpers

- Moved background rendering functionality from the editor to a dedicated GUI module, enhancing modularity and separation of concerns.
- Introduced layout helpers for consistent theme-aware sizing across the GUI, improving UI consistency and maintainability.
- Updated CMake configuration to reflect the new structure, ensuring proper linkage of the background renderer and layout helpers.

Benefits:
- Improved organization of GUI components, facilitating easier updates and enhancements.
- Enhanced user interface consistency through theme-aware layout management.

src/app/editor/ui/workspace_manager.h

@@ -47,6 +47,16 @@ class WorkspaceManager {
   void MaximizeCurrentWindow();
   void RestoreAllWindows();
   void CloseAllFloatingWindows();
+
+  // Window operations for keyboard navigation
+  void FocusNextWindow();
+  void FocusPreviousWindow();
+  void SplitWindowHorizontal();
+  void SplitWindowVertical();
+  void CloseCurrentWindow();
+
+  // Command execution (for WhichKey integration)
+  void ExecuteWorkspaceCommand(const std::string& command_id);
   
   // Session queries
   size_t GetActiveSessionCount() const;