feat(reminders): reminder notifications — EVENT_REMINDER receiver, onboarding step, settings toggle (v1.4)

Calendula now posts event reminders itself (the Etar model): the provider
schedules the alarms and broadcasts EVENT_REMINDER, but a calendar app must
turn them into visible notifications — essential for users whose only
calendar app this is. A manifest-registered, exported receiver (data scheme
content://com.android.calendar) wakes us at reminder time; no foreground
service, no own alarm scheduling.

Delivery path (data/reminders/): EventReminderReceiver (Hilt, goAsync) →
ReminderAlertStore queries CalendarAlerts for STATE_SCHEDULED rows with
ALARM_TIME <= now → ReminderNotifier posts one notification per alert on a
dedicated high-importance channel, then best-effort marks rows FIRED
(needs WRITE_CALENDAR; without it a re-broadcast silently replaces — tag
per alert + setOnlyAlertOnce). Swiped notifications never return: FIRED
rows are never re-queried, so no dismiss-intent machinery. Research
(AOSP CalendarAlarmManager): the provider creates alert rows only for
METHOD_ALERT reminders, so the email-reminder filter happens upstream.

Tapping opens the event's detail screen: MainActivity is singleTop now,
parses eventId/begin/end extras (onCreate + onNewIntent) into Compose
state, and CalendarHost consumes the key exactly like an event tap.

Onboarding gained a one-time second step after the calendar grant (shared
OnboardingScaffold extracted from PermissionScreen): explains delivery,
warns that a second calendar app with notifications on duplicates
reminders, requests POST_NOTIFICATIONS (dialog on API 33+ only; minSdk 29).
"Not now" turns the feature off; reminders default ON. Settings mirrors
the toggle in a new Notifications section with the duplicate hint, and
re-requests the permission when enabling. Strings DE+EN.

Deliberately deferred (roadmap): snooze/dismiss actions, BOOT_COMPLETED /
exact-alarm scheduling, battery-exemption prompts.

Tests: reminderTimeText (all-day UTC-midnight reading, exclusive end day,
midnight-crossing ranges), reminders/onboarding pref round-trips.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.planning/ROADMAP.md

@@ -65,7 +65,7 @@ guide here, not a contract — scope per slice is decided as we go.
 | v1.2 | Create event — form, FAB, last-used-calendar preselect | complete (shipped 2026-06-11) |
 | v1.2.1 | Form polish after on-device review — card design system, optional fields + settings defaults, OptionCard dialogs, expressive motion | complete (shipped 2026-06-11) |
 | v1.3 | Edit event — shared form, scoped recurring writes (this / following / all), recurrence picker | complete (shipped 2026-06-11) |
-| v1.4 | Reminder notifications — see below | planned |
+| v1.4 | Reminder notifications — see below | implemented (release awaits on-device review) |
 | v2.0 | Quick-add, occurrence edit, conflict dialog, polish, release | planned |
 
 ## v1.4 — Reminder Notifications