halkyon_app: instances, sign-in and sync plumbing for Halkyon's apps

What every Halkyon app's shell needs, taken from Halkyon Learn's and set
on halkyon_base's client: the instances a device keeps an account on,
the first start's tiles and where an instance keeps people's data, the
account switcher; signing in anonymously, with GitHub or an email code,
switching person and the start screen; the gates that build an app's
dependencies per instance and per person; and the sync service with its
status, Settings card and banner. Texts of its own in English and
German take the app's name; what only the app can say, it passes in.
Fakes for apps' tests are in testing.dart.

An instance's basics now name the app it serves (`app`, set by the
host's configuration), and an app accepts only an instance of its own.
SerialQueue and CoalescingRunner move into halkyon_core.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/CHANGELOG.md

@@ -0,0 +1,8 @@
+## 0.1.0
+
+* Halkyon Learn's instances, sign-in and sync plumbing, for every Halkyon app,
+  on an instance's `halkyon_base`: `InstanceChoice` and the first start,
+  `InstanceGate`, `SessionGate` and `PersonScope` over an app's
+  `AppDependencies`, `ServerpodAuthGateway` with its blocs and the email
+  sheet, `SyncService` with its status and widgets, texts of their own in
+  English and German that name the app (`AppFrame`), and a `testing` library.