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>

apps/learn/halkyon_learn_server/bin/main.dart

@@ -0,0 +1,7 @@
+import 'package:halkyon_learn_server/server.dart';
+
+/// This is the starting point for your Serverpod server. Typically, there is
+/// no need to modify this file.
+void main(List<String> args) {
+  run(args);
+}