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.

docs/plans/qt-frontend.md

@@ -113,4 +113,12 @@ C++ projects than GTK (which is C-based, though `gtkmm` exists).
    When the core calls `DrawText()`, `QtRenderer` should queue that
    command or draw directly to the widget's paint buffer.
 4. **Refactor `main.cc`** to instantiate `QApplication` instead of the
-   current manual loop.
+   current manual loop.
+
+---
+
+Note (2025-12): The Qt frontend defers all key processing to the
+existing command subsystem and keymaps, mirroring the ImGui path. There
+are no Qt-only keybindings; `QtInputHandler` translates Qt key events
+into the shared keymap flow (C-k prefix, Ctrl chords, ESC/Meta,
+universal-argument digits, printable insertion).