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/pubspec.yaml

@@ -7,6 +7,12 @@ environment:
 resolution: workspace
 
 dependencies:
+  # The client side of halkyon_base (ADR 0014), as the server's.
+  halkyon_base_client:
+    git:
+      url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
+      path: packages/halkyon_base_client
+      ref: af1de799f402ef76ad4513052c3e0008155a92fb
   serverpod_auth_idp_client: 4.0.4
   serverpod_client: 4.0.4
   serverpod_offline_sync_client: 0.0.10