chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/windows/runner/flutter_window.h

@@ -1,33 +0,0 @@
-#ifndef RUNNER_FLUTTER_WINDOW_H_
-#define RUNNER_FLUTTER_WINDOW_H_
-
-#include <flutter/dart_project.h>
-#include <flutter/flutter_view_controller.h>
-
-#include <memory>
-
-#include "win32_window.h"
-
-// A window that does nothing but host a Flutter view.
-class FlutterWindow : public Win32Window {
- public:
-  // Creates a new FlutterWindow hosting a Flutter view running |project|.
-  explicit FlutterWindow(const flutter::DartProject& project);
-  virtual ~FlutterWindow();
-
- protected:
-  // Win32Window:
-  bool OnCreate() override;
-  void OnDestroy() override;
-  LRESULT MessageHandler(HWND window, UINT const message, WPARAM const wparam,
-                         LPARAM const lparam) noexcept override;
-
- private:
-  // The project to run.
-  flutter::DartProject project_;
-
-  // The Flutter instance hosted by this window.
-  std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
-};
-
-#endif  // RUNNER_FLUTTER_WINDOW_H_