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/pubspec.yaml

@@ -0,0 +1,62 @@
+name: halkyon_app
+description: "What every Halkyon app is built around: the instances a device keeps accounts on, signing in through an instance's halkyon_base, each person's data opened and closed, and syncing it."
+version: 0.1.0
+publish_to: none
+resolution: workspace
+
+environment:
+  sdk: ^3.13.0
+  flutter: ^3.44.4
+
+dependencies:
+  clock: ^1.1.3
+  equatable: ^3.0.0
+  # The flag of where an instance keeps people's data; it draws lipis'
+  # flag-icons, whose MIT notice this package registers (assets/flags/).
+  flag: ^7.1.0
+  flutter:
+    sdk: flutter
+  flutter_bloc: ^9.1.1
+  # gen-l10n's output imports it; apps take Material's texts from material_ui.
+  flutter_localizations:
+    sdk: flutter
+  # LoopbackWebAuth takes the place of the package's own browser half on
+  # Linux and Windows.
+  flutter_web_auth_2_platform_interface: ^5.0.0
+  halkyon_base_client:
+    path: ../halkyon_base_client
+  halkyon_core:
+    path: ../halkyon_core
+  halkyon_ui:
+    path: ../halkyon_ui
+  # A client of halkyon_base alone can be given a fake one in tests.
+  http: ^1.6.0
+  intl: ^0.20.3
+  lucide_icons_flutter: ^3.1.20
+  material_ui: ^1.5.0
+  path: ^1.9.1
+  # Serverpod's packages move together, at one version (as the apps pin them).
+  serverpod_auth_core_client: 4.0.4
+  serverpod_auth_idp_client: 4.0.4
+  serverpod_auth_idp_flutter: 4.0.4
+  serverpod_client: 4.0.4
+  # Its sync refuses an app whose synced tables differ from the server's
+  # (OfflineSyncTablesHashMismatchException).
+  serverpod_offline_sync_client: 0.0.10
+  url_launcher: ^6.3.2
+  window_to_front: ^0.0.4
+
+dev_dependencies:
+  fake_async: ^1.3.3
+  flutter_test:
+    sdk: flutter
+  # A test drives sign-in through its front, as Serverpod's GitHub sign-in does.
+  flutter_web_auth_2: ^5.1.0
+
+flutter:
+  # Generates HalkyonAppLocalizations from lib/l10n (see l10n.yaml).
+  generate: true
+
+  assets:
+    # flag-icons' MIT notice, for the licence list (registerFlagIconsLicense).
+    - assets/flags/LICENSE-flag-icons.txt