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_flutter/pubspec.yaml

@@ -73,6 +73,14 @@ dependencies:
   # schedules.
   fsrs: 2.0.1
   go_router: ^18.0.2
+  # The client side of halkyon_base (ADR 0014): what the app calls to sign in
+  # and to manage the account, and the refusals it reads. As the server's,
+  # pinned to a signed commit of halkyon_shared's main.
+  halkyon_base_client:
+    git:
+      url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
+      path: packages/halkyon_base_client
+      ref: af1de799f402ef76ad4513052c3e0008155a92fb
   halkyon_learn_client:
     path: ../halkyon_learn_client
   # Halkyon's look, shared by its apps (halkyon_shared): the theme, the bundled Noto
@@ -83,7 +91,7 @@ dependencies:
     git:
       url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
       path: packages/halkyon_ui
-      ref: 80fa7b96286b7a00791c4b7ea42af408f4ea548a
+      ref: af1de799f402ef76ad4513052c3e0008155a92fb
   # Anki's fields are HTML; the editor brings this parser along already.
   html: ^0.15.6
   # Talks to Jellyfin servers; Serverpod's client brings it along already.