Remove the Qt frontend

The Qt GUI was unused: out of make-app-release since March, not built by
the gate, and its app bundle could not start (no platform plugin). Every
renderer and input change had to be made in it separately. kge is the
ImGui frontend only.

Removes QtFrontend/QtInputHandler/QtRenderer, the KTE_USE_QT option and
its #if branches (Command.cc, GUITheme.h, main.cc), the Nix qt package
and dev shell, the commented-out Qt release steps, the Qt frontend plan,
and Qt mentions in AGENTS.md, DEVELOPER_GUIDE.md, REWRITE.md and code
comments. Git history keeps the code.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

REWRITE.md

@@ -938,7 +938,7 @@ pub enum CommandError {
 **Current C++ Implementation:**
 
 - `Frontend` trait with Init/Step/Shutdown lifecycle
-- Three implementations: Terminal (ncurses), ImGui (SDL2/OpenGL), Qt
+- Two implementations: Terminal (ncurses), ImGui (SDL2/OpenGL)
 - Step() does one iteration: poll input, dispatch commands, render
 - Each frontend owns InputHandler and Renderer instances
 
@@ -986,7 +986,7 @@ pub enum FrontendError {
 - `InputHandler` trait with Poll() returning `MappedInput`
 - `MappedInput`: hasCommand bool, CommandId, arg string, count
 - Terminal: ncurses-based, translates keys to commands
-- ImGui/Qt: event-based, handles text input differently
+- ImGui: event-based, handles text input differently
 - Keymap lookups via `KKeymap` functions (KCommand, Ctrl, Esc)
 
 **Rust Design:**
@@ -1075,7 +1075,7 @@ impl InputHandler for TerminalInputHandler {
 
 - `Renderer` trait with single Draw(Editor&) method
 - Terminal: ncurses-based, manual cursor positioning
-- ImGui/Qt: retained mode GUI, draw calls within Begin/End blocks
+- ImGui: immediate mode GUI, draw calls within Begin/End blocks
 - Syntax highlighting integrated via Buffer::Highlighter()
 
 **Rust Design:**
@@ -1200,7 +1200,7 @@ impl Frontend for TerminalFrontend {
 
 #### 9.2 GUI Frontend (egui-based)
 
-For GUI, consider replacing ImGui/Qt with `egui` (pure Rust, immediate
+For GUI, consider replacing ImGui with `egui` (pure Rust, immediate
 mode):
 
 ```rust