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/dart_test.yaml

@@ -1,5 +0,0 @@
-tags:
-  # Tests against a running server (`serverpod start`), such as the sync tests in
-  # test/sync/ (ADR 0001, rule 9). tool/check.sh skips them; run them with
-  # `flutter test -t requires-server`.
-  requires-server: {}