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';