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/analysis_options.yaml

@@ -0,0 +1,11 @@
+# The workspace's lint rules (see ../../analysis_options.yaml).
+include: ../../analysis_options.yaml
+
+analyzer:
+  exclude:
+    - build/**
+    # Generated by gen-l10n (see l10n.yaml).
+    - lib/l10n/halkyon_app_localizations*.dart
+
+formatter:
+  trailing_commas: preserve