feat(app): choose whether studying shows the Pomodoro timer

Settings has a Study card: the Pomodoro profiles, and a switch "Show
the Pomodoro timer while studying" (on unless switched off). Like the
appearance, it is the device's own setting.

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_de.arb

@@ -287,6 +287,9 @@
   "settingsCitationStyleChicago": "Chicago (18. Auflage)",
   "settingsCitationPreview": "Vorschau",
   "settingsCitationGuide": "Offizieller Leitfaden",
+  "settingsStudy": "Lernen",
+  "settingsStudyPomodoroTitle": "Pomodoro-Timer beim Lernen anzeigen",
+  "settingsStudyPomodoroSubtitle": "Zeige den Fokus-/Pausen-Countdown in der Lernsitzung, damit du die verbleibende Zeit siehst, ohne die Benachrichtigung zu öffnen.",
   "settingsVersion": "Version {version} (Build {build})",
   "syncTitle": "Synchronisierung",
   "syncStateSyncing": "Wird synchronisiert…",