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_en.arb

@@ -312,6 +312,9 @@
   "settingsCitationStyleChicago": "Chicago (18th edition)",
   "settingsCitationPreview": "Preview",
   "settingsCitationGuide": "Official guide",
+  "settingsStudy": "Study",
+  "settingsStudyPomodoroTitle": "Show the Pomodoro timer while studying",
+  "settingsStudyPomodoroSubtitle": "Show the focus / break countdown inside the study session, so you can see the time left without opening the notification.",
   "settingsVersion": "Version {version} (build {build})",
   "@settingsVersion": {"placeholders": {"version": {"type": "String"}, "build": {"type": "String"}}},
   "syncTitle": "Sync",