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.

ImGuiFrontend.h

@@ -0,0 +1,36 @@
+/*
+ * GUIFrontend - couples ImGuiInputHandler + GUIRenderer and owns SDL2/ImGui lifecycle
+ */
+#pragma once
+#include "Frontend.h"
+#include "GUIConfig.h"
+#include "ImGuiInputHandler.h"
+#include "ImGuiRenderer.h"
+
+
+struct SDL_Window;
+typedef void *SDL_GLContext;
+
+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:
+	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;
+};