feat(reminders): allow multiple default reminders per calendar

Codeberg #14. Event-level multi-reminders already worked; this widens the
*defaults* layer from a single Int to a List<Int> so a calendar's default can
carry several lead times (e.g. a birthday calendar: one week before *and* on
the day).

- SettingsPrefs: global + per-calendar timed/all-day defaults become List<Int>;
  CalendarReminderOverride.Minutes(List<Int>); resolveDefaultReminder returns a
  list. Storage stays backward-compatible — a legacy single value ("30") parses
  to [30], comma-joined for multiples, "none"/empty for no reminder.
- ReminderDefaultPicker is now multi-select: M3 Checkbox rows over the existing
  grouped-tonal idiom, an exclusive "use default" group for per-calendar
  overrides, and a Custom row that adds an arbitrary lead time to the set.
  Optimistic local state so quick successive toggles don't race the settings
  flow round-trip.
- New events seed their reminder list from the resolved default.

Tests cover none/single/multiple round-trips, legacy single-value parsing, and
per-calendar list overrides. lint + test + assembleDebug green.

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

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

@@ -324,7 +324,6 @@
     <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_title">Per-calendar reminders</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>