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_base_server/test/integration/support/fixtures.dart

@@ -7,6 +7,9 @@ import '../test_tools/serverpod_test_tools.dart';
 /// The app the tests' host is.
 const testAppName = 'Halkyon Test';
 
+/// The id of the app the tests' host serves.
+const testApp = 'test';
+
 /// Where the tests' host says its project publishes its source.
 const testUpstreamSourceCodeUrl = 'https://code.example.org/halkyon_test';
 
@@ -26,6 +29,7 @@ void useBase(
   HalkyonBase.configure(
     pod,
     HalkyonBase(
+      app: testApp,
       appName: testAppName,
       upstreamSourceCodeUrl: testUpstreamSourceCodeUrl,
       minimumAppBuild: testMinimumAppBuild,