refactor(app): instances, sign-in and sync plumbing come from halkyon_app

The instances a device keeps an account on, the first start, the
account switcher, signing in and switching person, the gates that build
the dependencies per instance and per person, and the sync service with
its status, card and banner move to halkyon_app (halkyon_shared, ADR
0014), on halkyon_base's client. Learn supplies its frame (name, word,
icon, theme, launch screen, and the two texts only it can say), its
dependencies, whether the device holds content, and its world map's
country names; every text reads as before. An instance counts only
where its basics name the app `learn`, which learnBase now sets.

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_server/pubspec.yaml

@@ -20,7 +20,7 @@ dependencies:
     git:
       url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
       path: packages/halkyon_base_server
-      ref: af1de799f402ef76ad4513052c3e0008155a92fb
+      ref: e23cc77f7c30a2a07015237ef0f054d1d1ccad2b
   # Reads the pages profile links point to, for their rel="me" back-links.
   html: ^0.15.6
   # Mails email sign-in's codes over SMTP (ADR 0010).