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_client/README.md

@@ -0,0 +1,20 @@
+# halkyon_learn_client
+
+This is your Serverpod client. The code in here is mostly generated by
+Serverpod, but you may want to make changes if you are adding modules to your
+project.
+
+## Licence
+
+Copyright (C) 2026 The Halkyon Learn contributors
+
+This program is free software: you can redistribute it and/or modify it under the
+terms of the GNU Affero General Public License as published by the Free Software
+Foundation, either version 3 of the License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License along with
+this program (`LICENSE`). If not, see <https://www.gnu.org/licenses/>.