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/test/unit/auth/learn_base_test.dart

@@ -0,0 +1,9 @@
+import 'package:halkyon_learn_server/src/auth/learn_base.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("Learn's instances name the app `learn`, the only instances Learn's "
+      'apps accept', () {
+    expect(learnBase().app, 'learn');
+  });
+}