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.

QtInputHandler.h

@@ -0,0 +1,40 @@
+/*
+ * QtInputHandler - Qt-based input mapping for GUI mode
+ */
+#pragma once
+
+#include <mutex>
+#include <queue>
+
+#include "InputHandler.h"
+
+class QKeyEvent;
+
+class QtInputHandler final : public InputHandler {
+public:
+	QtInputHandler() = default;
+
+	~QtInputHandler() override = default;
+
+
+	void Attach(Editor *ed) override
+	{
+		ed_ = ed;
+	}
+
+
+	// Translate a Qt key event to editor command and enqueue if applicable.
+	// Returns true if it produced a mapped command or consumed input.
+	bool ProcessKeyEvent(const QKeyEvent &e);
+
+	bool Poll(MappedInput &out) override;
+
+private:
+	std::mutex mu_;
+	std::queue<MappedInput> q_;
+	bool k_prefix_                 = false;
+	bool k_ctrl_pending_           = false; // C-k C-… qualifier
+	bool esc_meta_                 = false; // ESC-prefix for next key
+	bool suppress_text_input_once_ = false; // reserved (Qt sends text in keyPressEvent)
+	Editor *ed_                    = nullptr;
+};