feat(todo): a task done ticks on phones and chimes on desktops

The developer's request of 10 Nov 2026: completing a task should feel like
something. `CompletionFeedback`, held by `TodoDevice` and handed to each
person's `TodoPerson`, gives a light haptic tick on Android and iOS and, on
desktops, a short quiet chime of our own: two soft bell tones a fifth apart,
synthesized by `apps/todo/tool/completion_sound.dart` into
`assets/sounds/task_done.wav`, so it needs no one's licence. A task's check
and the selection's Complete call it once, however many tasks are done;
an undo is silent.

Settings' Tasks turns the chime off on a desktop, kept per device in
`<app support>/completion_feedback.json`; a phone is offered no switch. A
device that can't play loses only the sound. The player is `audioplayers`,
as Learn's sounds: on Linux it links GStreamer, so Todo's snap gains the
`audio-playback` plug and GStreamer's headers to build, as Learn's recipe has.

Whats-New: Completing a task now gives a light tap on phones and a short chime on computers, which Settings › Tasks turns off.
Whats-New-De: Eine erledigte Aufgabe gibt jetzt auf dem Handy ein leichtes Tippen und am Computer einen kurzen Klang, den du unter Einstellungen › Aufgaben ausschaltest.

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/lib/l10n/app_de.arb

@@ -340,6 +340,8 @@
   "weekStartLanguage": "Wie in der Sprache",
   "resetSubTasksTitle": "Unteraufgaben zurücksetzen",
   "resetSubTasksHint": "Wieder öffnen, wenn eine wiederkehrende Aufgabe erledigt ist",
+  "completionSoundTitle": "Ton beim Erledigen",
+  "completionSoundHint": "Ein kurzer Klang, wenn du eine Aufgabe erledigst",
   "automaticReminderTitle": "Automatische Erinnerung",
   "automaticReminderHint": "Für eine Aufgabe mit Uhrzeit, wenn sie keine eigene Erinnerung hat",
   "automaticReminderNone": "Keine",

apps/todo/halkyon_todo_flutter/lib/l10n/app_en.arb

@@ -961,6 +961,14 @@
   "@resetSubTasksHint": {
     "description": "Settings of tasks and reminders (ADR 0033, D4)."
   },
+  "completionSoundTitle": "Sound when done",
+  "@completionSoundTitle": {
+    "description": "Settings of tasks, on a desktop alone: whether a short chime plays as a task is marked done (decided 10 Nov 2026)."
+  },
+  "completionSoundHint": "A short chime as a task is marked done",
+  "@completionSoundHint": {
+    "description": "Explains completionSoundTitle."
+  },
   "automaticReminderTitle": "Automatic reminder",
   "@automaticReminderTitle": {
     "description": "Settings of tasks and reminders (ADR 0033, D4)."

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations.dart

@@ -2140,6 +2140,18 @@ abstract class AppLocalizations {
   /// **'Open them again when a recurring task is done'**
   String get resetSubTasksHint;
 
+  /// Settings of tasks, on a desktop alone: whether a short chime plays as a task is marked done (decided 10 Nov 2026).
+  ///
+  /// In en, this message translates to:
+  /// **'Sound when done'**
+  String get completionSoundTitle;
+
+  /// Explains completionSoundTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'A short chime as a task is marked done'**
+  String get completionSoundHint;
+
   /// Settings of tasks and reminders (ADR 0033, D4).
   ///
   /// In en, this message translates to:

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_de.dart

@@ -1306,6 +1306,13 @@ class AppLocalizationsDe extends AppLocalizations {
   String get resetSubTasksHint =>
       'Wieder öffnen, wenn eine wiederkehrende Aufgabe erledigt ist';
 
+  @override
+  String get completionSoundTitle => 'Ton beim Erledigen';
+
+  @override
+  String get completionSoundHint =>
+      'Ein kurzer Klang, wenn du eine Aufgabe erledigst';
+
   @override
   String get automaticReminderTitle => 'Automatische Erinnerung';
 

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations_en.dart

@@ -1305,6 +1305,12 @@ class AppLocalizationsEn extends AppLocalizations {
   String get resetSubTasksHint =>
       'Open them again when a recurring task is done';
 
+  @override
+  String get completionSoundTitle => 'Sound when done';
+
+  @override
+  String get completionSoundHint => 'A short chime as a task is marked done';
+
   @override
   String get automaticReminderTitle => 'Automatic reminder';
 

apps/todo/halkyon_todo_flutter/lib/src/app/todo_dependencies.dart

@@ -33,6 +33,7 @@ import 'package:halkyon_todo_flutter/src/feature/section/data/section_repository
 import 'package:halkyon_todo_flutter/src/feature/section/data/sync_section_gateway.dart';
 import 'package:halkyon_todo_flutter/src/feature/settings/data/settings_repository.dart';
 import 'package:halkyon_todo_flutter/src/feature/settings/data/sync_settings_gateway.dart';
+import 'package:halkyon_todo_flutter/src/feature/task/data/completion_feedback.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/data/sync_task_gateway.dart';
 import 'package:halkyon_todo_flutter/src/feature/task/data/task_repository.dart';
 import 'package:halkyon_todo_flutter/src/feature/weather/data/weather_gateway.dart';
@@ -98,6 +99,10 @@ class TodoPerson(
   /// given; else for this session.
   final PaneWidths? _paneWidths,
 
+  /// What a task done feels like: the device's, where it is given; else
+  /// nothing.
+  final CompletionFeedback? _completionFeedback,
+
   /// The projects whose sub-projects are folded away (decided 10 Nov
   /// 2026): the person's folder's, where it is given; else for this session.
   final FoldedProjects? _foldedProjects,
@@ -118,6 +123,10 @@ class TodoPerson(
   /// How wide the device keeps its side panes (ADR 0033, D33).
   late final PaneWidths paneWidths = _paneWidths ?? PaneWidths();
 
+  /// A tick or a chime as a task is done (decided 10 Nov 2026).
+  late final CompletionFeedback completionFeedback =
+      _completionFeedback ?? CompletionFeedback();
+
   /// The projects folded in Browse and the sidebar, kept on the device.
   late final FoldedProjects foldedProjects =
       _foldedProjects ?? FoldedProjects();
@@ -364,6 +373,7 @@ class TodoDependencies({
             locationReminders: device.locationReminders,
             locationAccess: device.locationAccess,
             paneWidths: device.paneWidths,
+            completionFeedback: device.completionFeedback,
             onClosed: closed.complete,
           );
         } on Object {
@@ -405,6 +415,7 @@ Future<TodoPerson> _openPersonData(
   required LocationReminderGateway locationReminders,
   required LocationReminderAccess locationAccess,
   required PaneWidths paneWidths,
+  required CompletionFeedback completionFeedback,
   required void Function() onClosed,
 }) async {
   final db = await client.createSyncSession(
@@ -471,6 +482,7 @@ Future<TodoPerson> _openPersonData(
     locationReminders: locationReminders,
     locationAccess: locationAccess,
     paneWidths: paneWidths,
+    completionFeedback: completionFeedback,
     foldedProjects: await FoldedProjects.open(
       File(p.join(folder.path, 'folded_projects.json')),
     ),
@@ -522,6 +534,9 @@ class TodoDevice({
 
   /// How wide it keeps its side panes (D33).
   required final PaneWidths paneWidths,
+
+  /// What a task done feels like, the sound turned off or not.
+  required final CompletionFeedback completionFeedback,
 }) {
   /// Whether the device may show reminders, asked once an app start,
   /// whoever is signed in (D31).
@@ -542,6 +557,9 @@ class TodoDevice({
       appBuild: int.tryParse(package.buildNumber),
       appVersion: package.version,
       paneWidths: await PaneWidths.open(File(p.join(support, 'panes.json'))),
+      completionFeedback: await CompletionFeedback.forDevice(
+        File(p.join(support, 'completion_feedback.json')),
+      ),
       reminders: await _reminders(),
       locationReminders: Platform.isAndroid
           ? const AndroidLocationReminders()

apps/todo/halkyon_todo_flutter/lib/src/feature/task/data/completion_feedback.dart

@@ -0,0 +1,112 @@
+import 'dart:convert';
+import 'dart:developer';
+import 'dart:io';
+
+import 'package:audioplayers/audioplayers.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+/// What a task done feels like (decided by the developer, 10 Nov 2026): a
+/// light haptic tick on a phone, a short quiet chime of Todo's own on a
+/// desktop ([sounds]), which Settings turns off there. The device's matter,
+/// whoever is signed in, kept in [_file] where there is one; neither sealed
+/// nor synced.
+class CompletionFeedback({
+  final File? _file,
+
+  /// Whether this device chimes rather than ticks: a desktop's.
+  final bool sounds = false,
+
+  /// The tick; nothing in tests that don't say.
+  final Future<void> Function() _tick = _nothing,
+
+  /// The chime; nothing in tests that don't say.
+  final Future<void> Function() _chime = _nothing,
+}) extends ChangeNotifier {
+  var _soundOn = true;
+
+  /// Opens what [file] holds; the sound on where it can't be read.
+  static Future<CompletionFeedback> open(
+    File file, {
+    required bool sounds,
+    Future<void> Function() tick = _nothing,
+    Future<void> Function() chime = _nothing,
+  }) async {
+    final feedback = CompletionFeedback(
+      file: file,
+      sounds: sounds,
+      tick: tick,
+      chime: chime,
+    );
+    try {
+      if (file.existsSync()) {
+        final json = jsonDecode(await file.readAsString());
+        if (json is Map<String, Object?> && json['sound'] is bool) {
+          feedback._soundOn = json['sound']! as bool;
+        }
+      }
+    } on Object catch (error) {
+      log('Completion sound not read: ${error.runtimeType}', name: 'Tasks');
+    }
+    return feedback;
+  }
+
+  /// The device's own: a tick on Android and iOS, else the chime from
+  /// `assets/sounds/task_done.wav`.
+  static Future<CompletionFeedback> forDevice(File file) {
+    final phone = Platform.isAndroid || Platform.isIOS;
+    return open(
+      file,
+      sounds: !phone,
+      tick: HapticFeedback.lightImpact,
+      chime: phone ? _nothing : _AssetChime().play,
+    );
+  }
+
+  /// Whether Settings offers turning the sound off: on a desktop alone.
+  bool get offersSound => sounds;
+
+  /// Whether a desktop chimes; on at first.
+  bool get soundOn => _soundOn;
+
+  /// Turns the chime [on] or off, and keeps it.
+  Future<void> setSoundOn({required bool on}) async {
+    if (_soundOn == on) {
+      return;
+    }
+    _soundOn = on;
+    notifyListeners();
+    try {
+      await _file?.writeAsString(jsonEncode({'sound': on}));
+    } on Object catch (error) {
+      log('Completion sound not kept: ${error.runtimeType}', name: 'Tasks');
+    }
+  }
+
+  /// A task, or several at once, was just marked done: once whatever their
+  /// number. Never throws: a device that can't play loses only the sound.
+  Future<void> completed() async {
+    try {
+      if (!sounds) {
+        await _tick();
+      } else if (_soundOn) {
+        await _chime();
+      }
+    } on Object catch (error) {
+      log('No completion feedback: ${error.runtimeType}', name: 'Tasks');
+    }
+  }
+}
+
+Future<void> _nothing() async {}
+
+/// The chime on `audioplayers`, one player made at the first, kept for the
+/// app's life.
+class _AssetChime() {
+  late final _player = AudioPlayer();
+
+  Future<void> play() async {
+    await _player.stop();
+    await _player.play(AssetSource('sounds/task_done.wav'));
+  }
+}

apps/todo/halkyon_todo_flutter/lib/src/feature/task/widget/task_selection.dart

@@ -355,6 +355,12 @@ class _SelectionActions(
     final context = _context;
     final tasks = _tasks;
     final chosen = _chosen.where((task) => !task.content.completed).toList();
+    if (chosen.isNotEmpty) {
+      // Once for them all.
+      unawaited(
+        PersonScope.of<TodoPerson>(context).completionFeedback.completed(),
+      );
+    }
     final reopen = context.read<TodoCubit>().state.settings.tasks.resetSubTasks;
     final now = clock.now();
     final before = await _write(

apps/todo/halkyon_todo_flutter/lib/src/feature/task/widget/task_tile.dart

@@ -386,9 +386,14 @@ class const TaskCheck({required final OpenTask task, super.key})
         color: color,
       ),
       onPressed: () async {
-        final tasks = PersonScope.of<TodoPerson>(context).tasks;
+        final person = PersonScope.of<TodoPerson>(context);
+        final tasks = person.tasks;
         final stored = StoredTask(task.id, content);
         final done = !content.completed;
+        if (done) {
+          // Felt as the tap lands, not once it is written.
+          unawaited(person.completionFeedback.completed());
+        }
         final reopen = context
             .read<TodoCubit>()
             .state

apps/todo/halkyon_todo_flutter/lib/src/settings/todo_settings_cards.dart

@@ -75,11 +75,37 @@ class const TaskSettingsCard({super.key}) extends StatelessWidget {
             settings.copyWith(tasks: tasks.copyWith(resetSubTasks: on)),
           ),
         ),
+        const _CompletionSound(),
       ],
     );
   }
 }
 
+/// Whether a desktop chimes as a task is done (decided 10 Nov 2026): the
+/// device's, whoever is signed in; nothing on a phone, which ticks.
+class const _CompletionSound() extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    final feedback = PersonScope.of<TodoPerson>(context).completionFeedback;
+    if (!feedback.offersSound) {
+      return const SizedBox.shrink();
+    }
+    return ListenableBuilder(
+      listenable: feedback,
+      builder: (context, _) => SwitchListTile(
+        secondary: const Icon(LucideIcons.volume2),
+        title: ExplainedTitle(
+          l10n.completionSoundTitle,
+          help: l10n.completionSoundHint,
+        ),
+        value: feedback.soundOn,
+        onChanged: (on) => feedback.setSoundOn(on: on),
+      ),
+    );
+  }
+}
+
 /// Settings' *Reminders* (D4): the automatic reminder a task given a time
 /// gets where it has none of its own (none at first), and how long a
 /// reminder is snoozed for from its notification; above them, while