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.

flake.nix

@@ -15,6 +15,7 @@
         full = kge;
         kte = (pkgsFor system).callPackage ./default.nix { graphical = false; };
         kge = (pkgsFor system).callPackage ./default.nix { graphical = true; };
+        qt  = (pkgsFor system).callPackage ./default.nix { graphical-qt = true; }
       });
     };
 }