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.

ImGuiInputHandler.h

@@ -0,0 +1,44 @@
+/*
+ * ImGuiInputHandler - ImGui/SDL2-based input mapping for GUI mode
+ */
+#pragma once
+#include <mutex>
+#include <queue>
+
+#include "InputHandler.h"
+
+
+union SDL_Event; // fwd decl to avoid including SDL here (SDL defines SDL_Event as a union)
+
+class ImGuiInputHandler final : public InputHandler {
+public:
+	ImGuiInputHandler() = default;
+
+	~ImGuiInputHandler() override = default;
+
+
+	void Attach(Editor *ed) override
+	{
+		ed_ = ed;
+	}
+
+
+	// Translate an SDL event to editor command and enqueue if applicable.
+	// Returns true if it produced a mapped command or consumed input.
+	bool ProcessSDLEvent(const SDL_Event &e);
+
+	bool Poll(MappedInput &out) override;
+
+private:
+	std::mutex mu_;
+	std::queue<MappedInput> q_;
+	bool k_prefix_       = false;
+	bool k_ctrl_pending_ = false; // if true, next k-suffix is treated as Ctrl- (qualifier via literal 'C' or '^')
+	// Treat ESC as a Meta prefix: next key is looked up via ESC keymap
+	bool esc_meta_ = false;
+	// When a printable keydown generated a non-text command, suppress the very next SDL_TEXTINPUT
+	// event produced by SDL for the same keystroke to avoid inserting stray characters.
+	bool suppress_text_input_once_ = false;
+
+	Editor *ed_ = nullptr; // attached editor for editor-owned uarg handling
+};