feat(reminders): configurable all-day reminder fire time

All-day events live at UTC midnight, so a raw "1 day before" reminder
fires at an off hour (02:00 local in CEST) rather than the morning. Add a
global "all-day reminder time" setting (default 09:00) and encode it into
the provider MINUTES offset so the reminder lands at the chosen wall-clock
time the day before instead.

- AllDayReminderEncoding: pure to/from provider-minutes helpers, keeping
  the form/UI/diff in whole-day "semantic" minutes and converting only at
  the Reminders read/write boundary (insertEvent, reconcileReminders,
  EventDetailMapper). Covers DST, negative offsets, and pre-existing rows.
- SettingsPrefs.allDayReminderTimeMinutes (default 540) threaded from the
  repository into the data-source write paths.
- Settings: a time-picker row, plus a shared TimePickerAlert lifted from
  the event editor.
- Fix the time picker's 12/24-hour detection: honour an explicit system
  override, else fall back to the device locale rather than the app's
  per-app language, so it matches the rest of the device.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -245,6 +245,20 @@
     <string name="settings_section_notifications">Notifications</string>
     <string name="settings_reminders">Event reminders</string>
     <string name="settings_reminders_hint">Seeing reminders twice? Another calendar app is posting them too — turn them off in one of the two.</string>
+    <string name="settings_default_reminder">Default reminder</string>
+    <string name="settings_default_reminder_allday">All-day events</string>
+    <string name="settings_allday_reminder_time">All-day reminder time</string>
+    <string name="settings_allday_reminder_time_hint">Reminders for all-day events fire at %1$s</string>
+    <string name="reminder_none">None</string>
+    <string name="reminder_use_default">Use default reminder</string>
+    <string name="reminder_custom_amount">Amount</string>
+    <string name="reminder_custom_with_value">Custom (%1$s)</string>
+    <string name="reminder_custom_set">Set</string>
+    <string name="settings_calendar_reminders_hint">Override the default per calendar — separately for timed and all-day events. A calendar can keep the default, drop it, or set its own.</string>
+    <string name="settings_calendar_reminder_inherits">Default (%1$s)</string>
+    <string name="settings_reliable_delivery">Reliable delivery</string>
+    <string name="settings_reliable_delivery_hint">Android may delay reminders to save battery. Exempt Calendula so they arrive on time.</string>
+    <string name="settings_reliable_delivery_exempt">Exempt from battery optimisation — reminders arrive on time.</string>
     <string name="settings_section_calendars">Calendars</string>
     <string name="settings_manage_calendars">Manage calendars</string>
     <string name="settings_manage_calendars_hint">Create local calendars; manage synced ones</string>