Add multi-window support to GUI with shared buffer list and improved input handling

- Introduced support for multiple windows, sharing the primary editor's buffer list.
- Added `GUIFrontend::OpenNewWindow_` for creating secondary windows with independent dimensions and input handlers.
- Redesigned `WindowState` to encapsulate per-window attributes (dimensions, renderer, input, etc.).
- Updated input processing and command execution to route events based on active window, preserving window-level states.
- Enhanced SDL2 and ImGui integration for proper context management across multiple windows.
- Increased robustness by handling window closing, resizing, and cleanup of secondary windows without affecting the primary editor.
- Updated documentation and key bindings for multi-window operations (e.g., Cmd+N / Ctrl+Shift+N).
- Version updated to 1.8.0 to reflect the major GUI enhancement.

ImGuiFrontend.h

@@ -2,10 +2,14 @@
  * GUIFrontend - couples ImGuiInputHandler + GUIRenderer and owns SDL2/ImGui lifecycle
  */
 #pragma once
+#include <memory>
+#include <vector>
+
 #include "Frontend.h"
 #include "GUIConfig.h"
 #include "ImGuiInputHandler.h"
 #include "ImGuiRenderer.h"
+#include "Editor.h"
 
 
 struct SDL_Window;
@@ -24,13 +28,25 @@ public:
 	void Shutdown() override;
 
 private:
+	// Per-window state
+	struct WindowState {
+		SDL_Window *window   = nullptr;
+		SDL_GLContext gl_ctx = nullptr;
+		ImGuiInputHandler input{};
+		ImGuiRenderer renderer{};
+		Editor editor{};
+		int width  = 1280;
+		int height = 800;
+		bool alive = true;
+	};
+
+	// Open a new secondary window sharing the primary editor's buffer list.
+	// Returns false if window creation fails.
+	bool OpenNewWindow_(Editor &primary);
+
 	static bool LoadGuiFont_(const char *path, float size_px);
 
 	GUIConfig config_{};
-	ImGuiInputHandler input_{};
-	ImGuiRenderer renderer_{};
-	SDL_Window *window_   = nullptr;
-	SDL_GLContext gl_ctx_ = nullptr;
-	int width_            = 1280;
-	int height_           = 800;
-};
+	// Primary window (index 0 in windows_); created during Init.
+	std::vector<std::unique_ptr<WindowState> > windows_;
+};