feat(todo): the app on halkyon_app, with its own icon

Todo's app is halkyon_app's instance and session gates with Todo's
frame ("Todo" under "Halkyon", its icon, its own sentences) and an
instance check accepting only `todo`; TodoDependencies on an instance,
an empty TodoPerson per person, nothing kept or synced yet. A person
sees an empty home and their account: whose it is on which instance,
switching account, signing out and deleting it (each asking first where
it deletes), and About with the source of what the instance runs and
the licence list. Texts in British English and German. App ID
app.halkyon.todo on every platform, GitHub's callback scheme registered
on Android, no web build.

Its icon is Lucide's list-checks (lucide-static 1.50.0) in the brand's
blues on white: tool/app_icons.py draws its strokes with Pillow and
writes every platform's launcher icon; Lucide's ISC notice joins the
licence list.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/test/l10n/arb_files_test.dart

@@ -0,0 +1,78 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// The messages of the `.arb` file [name] in `lib/l10n/`, without metadata.
+Map<String, String> _messages(String name) => {
+  for (final MapEntry(:key, :value) in (jsonDecode(
+    File('lib/l10n/$name').readAsStringSync(),
+  ) as Map<String, Object?>).entries)
+    if (!key.startsWith('@')) key: value! as String,
+};
+
+/// The names of the placeholders in [message], with plural and select
+/// arguments; the words of their branches don't count. (gen-l10n reads
+/// quotes as text here: `use-escaping` is off.)
+Set<String> _placeholders(String message) {
+  final names = <String>{};
+  // Even depths hold text and placeholders, odd ones a plural's branches.
+  var depth = 0;
+  for (var i = 0; i < message.length; i++) {
+    switch (message[i]) {
+      case '{':
+        final name = RegExp(r'\{\s*(\w+)\s*[,}]').matchAsPrefix(message, i);
+        if (name != null && depth.isEven) {
+          names.add(name.group(1)!);
+        }
+        depth++;
+      case '}':
+        depth--;
+    }
+  }
+  return names;
+}
+
+void main() {
+  final template = _messages('app_en.arb');
+
+  test('German has every message, with the same placeholders', () {
+    final german = _messages('app_de.arb');
+
+    expect(
+      template.keys.where((key) => !german.containsKey(key)),
+      isEmpty,
+      reason: 'untranslated',
+    );
+    expect(
+      german.keys.where((key) => !template.containsKey(key)),
+      isEmpty,
+      reason: 'not in the template',
+    );
+    for (final MapEntry(:key, :value) in german.entries) {
+      expect(_placeholders(value), _placeholders(template[key]!), reason: key);
+    }
+  });
+
+  test("the English variants override the template's messages only, with "
+      'the same placeholders', () {
+    for (final variant in ['app_en_GB.arb', 'app_en_US.arb']) {
+      for (final MapEntry(:key, :value) in _messages(variant).entries) {
+        expect(template, contains(key), reason: '$variant: $key');
+        expect(
+          _placeholders(value),
+          _placeholders(template[key]!),
+          reason: '$variant: $key',
+        );
+      }
+    }
+  });
+
+  test('a placeholder is found also inside a plural', () {
+    expect(
+      _placeholders('{count, plural, =1{1 card} other{{count} cards in {t}}}'),
+      {'count', 't'},
+    );
+    expect(_placeholders("Couldn't open {name}"), {'name'});
+  });
+}