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/snap/snapcraft.yaml

@@ -53,14 +53,16 @@ apps:
     # How the session is kept: see snap/local/secret-backend (Learn's, kept in the
     # clone).
     command-chain: [bin/secret-backend]
-    # GTK, WebKitGTK (the sign-in's page) and libsecret come from the GNOME runtime,
-    # the reminders' notifications through the desktop's own; files and the browser
-    # go through the desktop's portals.
+    # GTK, WebKitGTK (the sign-in's page), GStreamer (the chime of a task done) and
+    # libsecret come from the GNOME runtime, the reminders' notifications through the
+    # desktop's own; files and the browser go through the desktop's portals.
     extensions: [gnome]
     plugs:
       - network
       # SimpleLogin's sign-in comes back on a loopback address the app listens on.
       - network-bind
+      # The chime of a task done (decided 10 Nov 2026).
+      - audio-playback
       # Never connected by itself; whoever connects it by hand gets the keyring.
       - password-manager-service
       # As Learn's: the sync's connection ended at once when the network goes;
@@ -101,9 +103,13 @@ parts:
     plugin: nil
     source: .
     source-type: git
-    # sodium's build hook compiles libsodium with its configure script and make
-    # (ADR 0017), with the Flutter part's clang.
+    # The chime's player (audioplayers) links GStreamer, which the GNOME SDK
+    # lacks; the GNOME runtime brings the same GStreamer to run, as in Learn's
+    # recipe. sodium's build hook compiles libsodium with its configure script and
+    # make (ADR 0017), with the Flutter part's clang.
     build-packages:
+      - libgstreamer1.0-dev
+      - libgstreamer-plugins-base1.0-dev
       - make
     override-build: |
       set -eu