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/test/instance/flag_icons_license_test.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  test("the flags' MIT notice joins the licence list, as their licence "
+      'asks', () async {
+    registerFlagIconsLicense();
+
+    final notices = [
+      await for (final entry in LicenseRegistry.licenses)
+        if (entry.packages.contains('flag-icons'))
+          entry.paragraphs.map((paragraph) => paragraph.text).join('\n'),
+    ];
+    expect(notices, hasLength(1));
+    expect(notices.single, contains('Copyright (c) 2013 Panayiotis Lipiridis'));
+    expect(
+      notices.single,
+      contains(
+        'The above copyright notice and this permission notice shall be '
+        'included',
+      ),
+    );
+  });
+}