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>

cmake/fix_bundle.cmake

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.15)
 
-# Fix up a macOS .app bundle by copying non-Qt dylibs into
+# Fix up a macOS .app bundle by copying non-system dylibs into
 # Contents/Frameworks and rewriting install names to use @rpath/@loader_path.
 #
 # Usage:
@@ -29,9 +29,8 @@ set(DIRS
 )
 
 # Note: We pass empty plugin list so fixup_bundle scans the executable and all
-# libs it references recursively. Qt frameworks already live in the bundle after
-# macdeployqt; this step is primarily for non-Qt dylibs (glib, icu, pcre2, zstd,
-# dbus, etc.).
+# libs it references recursively, copying every non-system dylib (e.g. Homebrew
+# SDL2, freetype, pcre2) into Contents/Frameworks; SDL3 is handled below.
 # fixup_bundle often fails if copied libraries are read-only.
 # We also try to use the system install_name_tool and otool to avoid issues with Anaconda's version.
 # Note: BundleUtilities uses find_program(gp_otool "otool") internally, so we might need to set it differently.