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/pubspec.yaml

@@ -13,6 +13,9 @@ environment:
 resolution: workspace
 
 dependencies:
+  # The chime of a task done on a desktop (decided 10 Nov 2026), as Learn's
+  # sounds: GStreamer on Linux.
+  audioplayers: ^6.8.1
   clock: ^1.1.3
   # Reactions on comments (ADR 0033, D36): every emoji, searched, recents on
   # the device.
@@ -101,6 +104,9 @@ flutter:
     # (<version>.<language>.txt), drafted by tool/release.dart and edited
     # (Train's ADR 0030, D61): halkyon_app's What's new shows them.
     - assets/whats_new/
+    # The chime of a task done on a desktop, Todo's own, synthesized by
+    # ../tool/completion_sound.dart.
+    - assets/sounds/task_done.wav
     # The app's icon, Lucide's list-todo (ADR 0031), and Lucide's licence for the
     # licence list (the repository's tool/app_icons.py writes both).
     - assets/icon/halkyon_todo_icon.png