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>

CMakeLists.txt

@@ -9,7 +9,6 @@ set(KTE_VERSION "1.12.0")
 # Default to terminal-only build to avoid SDL/OpenGL dependency by default.
 # Enable with -DBUILD_GUI=ON when SDL2/OpenGL/Freetype are available.
 set(BUILD_GUI ON CACHE BOOL "Enable building the graphical version.")
-set(KTE_USE_QT OFF CACHE BOOL "Build the QT frontend instead of ImGui.")
 set(BUILD_TESTS ON CACHE BOOL "Enable building test programs.")
 set(KTE_FONT_SIZE "18.0" CACHE STRING "Default font size for GUI")
 option(KTE_UNDO_DEBUG "Enable undo instrumentation logs" OFF)
@@ -163,25 +162,13 @@ if (BUILD_GUI)
     set(GUI_SOURCES
             GUIConfig.cc
     )
-    if (KTE_USE_QT)
-        find_package(Qt6 COMPONENTS Widgets REQUIRED)
-        set(GUI_SOURCES
-                ${GUI_SOURCES}
-                QtFrontend.cc
-                QtInputHandler.cc
-                QtRenderer.cc
-        )
-        # Expose preprocessor switch so sources can exclude ImGui-specific code
-        add_compile_definitions(KTE_USE_QT)
-    else ()
-        set(GUI_SOURCES
-                ${GUI_SOURCES}
-                ${FONT_SOURCES}
-                ImGuiFrontend.cc
-                ImGuiInputHandler.cc
-                ImGuiRenderer.cc
-        )
-    endif ()
+    set(GUI_SOURCES
+            ${GUI_SOURCES}
+            ${FONT_SOURCES}
+            ImGuiFrontend.cc
+            ImGuiInputHandler.cc
+            ImGuiRenderer.cc
+    )
 endif ()
 
 set(COMMON_SOURCES
@@ -311,24 +298,15 @@ if (BUILD_GUI)
             GUIConfig.h
     )
 
-    if (KTE_USE_QT)
-        set(GUI_HEADERS
-                ${GUI_HEADERS}
-                QtFrontend.h
-                QtInputHandler.h
-                QtRenderer.h
-        )
-    else ()
-        set(GUI_HEADERS
-                ${GUI_HEADERS}
-                ${THEME_HEADERS}
-                ${FONT_HEADERS}
-                ImGuiFrontend.h
-                ImGuiInputHandler.h
-                ImGuiRenderer.h
-                fonts/BerkeleyMono.h
-        )
-    endif ()
+    set(GUI_HEADERS
+            ${GUI_HEADERS}
+            ${THEME_HEADERS}
+            ${FONT_HEADERS}
+            ImGuiFrontend.h
+            ImGuiInputHandler.h
+            ImGuiRenderer.h
+            fonts/BerkeleyMono.h
+    )
 endif ()
 
 # kte (terminal-first) executable
@@ -473,17 +451,10 @@ if (BUILD_GUI)
 
     )
     target_compile_definitions(kge PRIVATE KTE_BUILD_GUI=1 KTE_DEFAULT_GUI=1 KTE_FONT_SIZE=${KTE_FONT_SIZE})
-    if (KTE_USE_QT)
-        target_compile_definitions(kge PRIVATE KTE_USE_QT=1)
-    endif ()
     if (KTE_UNDO_DEBUG)
         target_compile_definitions(kge PRIVATE KTE_UNDO_DEBUG=1)
     endif ()
-    if (KTE_USE_QT)
-        target_link_libraries(kge ${CURSES_LIBRARIES} Qt6::Widgets)
-    else ()
-        target_link_libraries(kge ${CURSES_LIBRARIES} imgui)
-    endif ()
+    target_link_libraries(kge ${CURSES_LIBRARIES} imgui)
 
     # Static linking on Linux only (macOS does not support static linking of system libraries)
     if (NOT APPLE AND KTE_STATIC_LINK)
@@ -565,7 +536,8 @@ if (BUILD_GUI)
 
     # Optional post-build bundle fixup (can also be run from scripts).
     # This provides a CMake target to run BundleUtilities' fixup_bundle on the
-    # built app, useful after macdeployqt to ensure non-Qt dylibs are internalized.
+    # built app, copying non-system dylibs (e.g. Homebrew SDL2/SDL3, freetype,
+    # pcre2) into the bundle so it runs on machines without them installed.
     if (APPLE AND TARGET kge)
         get_target_property(IS_BUNDLE kge MACOSX_BUNDLE)
         if (IS_BUNDLE)
@@ -573,7 +545,7 @@ if (BUILD_GUI)
                     COMMAND ${CMAKE_COMMAND}
                     -DAPP_BUNDLE=${CMAKE_CURRENT_BINARY_DIR}/$<TARGET_PROPERTY:kge,MACOSX_BUNDLE_BUNDLE_NAME>.app
                     -P ${CMAKE_CURRENT_LIST_DIR}/cmake/fix_bundle.cmake
-                    COMMENT "Running fixup_bundle on kge.app to internalize non-Qt dylibs"
+                    COMMENT "Running fixup_bundle on kge.app to internalize non-system dylibs"
                     VERBATIM)
             add_dependencies(kge_fixup_bundle kge)
         endif ()