Introduce QtFrontend with renderer, input handler, and theming support.

- Added `QtFrontend`, `QtRenderer`, and `QtInputHandler` for Qt-based UI rendering and input handling.
- Implemented support for theming, font customization, and palette overrides in GUITheme.
- Renamed and refactored ImGui-specific components (e.g., `GUIRenderer` -> `ImGuiRenderer`).
- Added cross-frontend integration for commands and visual font picker.

QtFrontend.h

@@ -0,0 +1,36 @@
+/*
+ * QtFrontend - couples QtInputHandler + QtRenderer and owns Qt lifecycle
+ */
+#pragma once
+
+#include "Frontend.h"
+#include "GUIConfig.h"
+#include "QtInputHandler.h"
+#include "QtRenderer.h"
+
+class QApplication;
+class QWidget;
+
+// Keep the public class name GUIFrontend to match main.cc selection logic.
+class GUIFrontend final : public Frontend {
+public:
+	GUIFrontend() = default;
+
+	~GUIFrontend() override = default;
+
+	bool Init(Editor &ed) override;
+
+	void Step(Editor &ed, bool &running) override;
+
+	void Shutdown() override;
+
+private:
+	GUIConfig config_{};
+	QtInputHandler input_{};
+	QtRenderer renderer_{};
+
+	QApplication *app_ = nullptr; // owned
+	QWidget *window_   = nullptr; // owned
+	int width_         = 1280;
+	int height_        = 800;
+};