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.

Command.h

@@ -27,6 +27,8 @@ enum class CommandId {
 	SearchReplace, // begin search & replace (two-step prompt)
 	OpenFileStart, // begin open-file prompt
 	VisualFilePickerToggle,
+	// GUI-only: toggle/show a visual font selector dialog
+	VisualFontPickerToggle,
 	// Buffers
 	BufferSwitchStart, // begin buffer switch prompt
 	BufferClose,