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>

docs/DEVELOPER_GUIDE.md

@@ -20,7 +20,7 @@ kte follows a clean separation of concerns with three main layers:
 
 ```
 ┌─────────────────────────────────────────┐
-│  Frontend Layer (Terminal/ImGui/Qt)     │
+│  Frontend Layer (Terminal/ImGui)        │
 │  - TerminalFrontend / ImGuiFrontend     │
 │  - InputHandler + Renderer interfaces   │
 └─────────────────────────────────────────┘
@@ -154,7 +154,6 @@ Implementations:
   `TerminalInputHandler`, `TerminalRenderer`)
 - **ImGui**: Dear ImGui-based (`ImGuiFrontend`, `ImGuiInputHandler`,
   `ImGuiRenderer`)
-- **Qt**: Qt-based (`QtFrontend`, `QtInputHandler`, `QtRenderer`)
 - **Test**: Programmatic testing (`TestFrontend`, `TestInputHandler`,
   `TestRenderer`)
 
@@ -196,7 +195,6 @@ Large files that may need attention:
   into logical groups
 - `Swap.cc` (1300 lines) - Crash recovery system (migrated to direct
   PieceTable operations)
-- `QtFrontend.cc` (985 lines) - Qt integration
 - `ImGuiRenderer.cc` (930 lines) - ImGui rendering
 - `PieceTable.cc` (800 lines) - Core data structure
 - `Buffer.cc` (763 lines) - Document model