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/macos/Runner/MainFlutterWindow.swift

@@ -1,15 +0,0 @@
-import Cocoa
-import FlutterMacOS
-
-class MainFlutterWindow: NSWindow {
-  override func awakeFromNib() {
-    let flutterViewController = FlutterViewController()
-    let windowFrame = self.frame
-    self.contentViewController = flutterViewController
-    self.setFrame(windowFrame, display: true)
-
-    RegisterGeneratedPlugins(registry: flutterViewController)
-
-    super.awakeFromNib()
-  }
-}