fix(app): the Pomodoro timer holds up with the app gone, asleep or edited

A review of the timer found:

- Stop in the notification with the app swiped away took the timer
  away there, but the app showed it again at its next start. The
  service now leaves a mark the app takes when it starts, and the timer
  it kept is forgotten.
- Pause, Resume and Skip did nothing with the app gone: since Android
  12 a service may not start an activity from a notification's button.
  The notification is posted again with buttons that open the app.
- A timer kept while the app was closed was shown as it was before it
  caught up, so its ended phase alerted a second time. It is now caught
  up before it first shows.
- Dart's timers sleep with the device, so a phase that ended while the
  phone or laptop slept went unnoticed until the app came back. While a
  phase runs the timer looks every 15 seconds, and on Android the
  alarm's receiver wakes it too.
- On Linux, changes that came at once overlapped on the bus (an
  indicator registered twice, notifications left over, a ticking label
  never stopped); they now go one at a time. The indicator registers
  again when GNOME's watcher restarts, and with a watcher that only
  turns up later.
- Changing the running profile could put the timer outside its cycle
  ("Focus 6 of 4", the Live Update's bar past its end); where it is
  stays within the new cycle, and a phase waiting at its full length
  takes the new length. A countdown past its end shows 00:00.
- Writes of the kept timer go one at a time, and a closed timer arms no
  more timers.
- The alert no longer renames the timer's notification channel, and a
  German hint said the alert came at the device's end.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -2962,7 +2962,7 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get pomodoroDoNotDisturbHint =>
-      'Das Gerät bleibt still, während du dich konzentrierst; die Meldung an seinem Ende ist trotzdem zu hören.';
+      'Das Gerät bleibt still, während du dich konzentrierst; die Meldung am Ende des Fokus ist trotzdem zu hören.';
 
   @override
   String get pomodoroDoNotDisturbDenied =>