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/windows/flutter/generated_plugin_registrant.cc

@@ -6,6 +6,7 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <audioplayers_windows/audioplayers_windows_plugin.h>
 #include <connectivity_plus/connectivity_plus_windows_plugin.h>
 #include <desktop_webview_window/desktop_webview_window_plugin.h>
 #include <emoji_picker_flutter/emoji_picker_flutter_plugin_c_api.h>
@@ -16,6 +17,8 @@
 #include <window_to_front/window_to_front_plugin.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
+  AudioplayersWindowsPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("AudioplayersWindowsPlugin"));
   ConnectivityPlusWindowsPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("ConnectivityPlusWindowsPlugin"));
   DesktopWebviewWindowPluginRegisterWithRegistrar(

apps/todo/halkyon_todo_flutter/windows/flutter/generated_plugins.cmake

@@ -3,6 +3,7 @@
 #
 
 list(APPEND FLUTTER_PLUGIN_LIST
+  audioplayers_windows
   connectivity_plus
   desktop_webview_window
   emoji_picker_flutter