refactor(auth)!: sign-in and the account come from halkyon_base

Sign-in (anonymous, GitHub, email codes), an account's ways in and its
deletion, the pruning and the instance's basics move into the
halkyon_base module (ADR 0014), which every Halkyon server includes.
Learn configures it in server.dart and deletes its own data with a
person through the module's hook, in the order it did. The identity
tables move unchanged: the migration records versions only. The app
calls the module's endpoints; instance.info() stays as it was.

BREAKING CHANGE: the sign-in endpoints moved into the module, so apps up
to build 20 can no longer sign in or start; minimumAppBuild is 21.
Deploy this only together with the release of build 21.

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_client/lib/migrations/migration_registry.dart

@@ -15,6 +15,7 @@ part '20261007205440868-card-buried-at/migration.dart';
 part '20261008165051407-pdf-position/migration.dart';
 part '20261009224606274-feedback/migration.dart';
 part '20261012162411835-studyset-deleted-at/migration.dart';
+part '20261019121406578-halkyon-base/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -32,6 +33,7 @@ class MigrationRegistry {
     _Migration20261008165051407_pdf_position(),
     _Migration20261009224606274_feedback(),
     _Migration20261012162411835_studyset_deleted_at(),
+    _Migration20261019121406578_halkyon_base(),
   ];
 
   /// List of all client-side database migration versions.