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/l10n.yaml

@@ -0,0 +1,14 @@
+# Flutter's gen-l10n: the strings in lib/l10n/*.arb become HalkyonAppLocalizations,
+# generated into lib/l10n/halkyon_app_localizations*.dart (committed, never edited or
+# formatted by hand, as apps take this package by git). halkyon_app_en.arb, the
+# template, is British English, also the fallback; halkyon_app_en_US.arb overrides
+# what American English spells differently; halkyon_app_en_GB.arb is empty; German
+# says "du". Where a text names the app, it takes the app's name as `{app}`.
+arb-dir: lib/l10n
+template-arb-file: halkyon_app_en.arb
+output-dir: lib/l10n
+output-localization-file: halkyon_app_localizations.dart
+output-class: HalkyonAppLocalizations
+# `HalkyonAppLocalizations.of(context).foo` without a `!`: apps install the
+# delegate.
+nullable-getter: false