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",