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>

CHANGELOG.md

@@ -7,6 +7,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Reminder notifications (v1.4): Calendula now delivers event reminders as
+  notifications itself — the system schedules them but posts nothing, so a
+  calendar app must (essential when Calendula is the only one installed).
+  Due reminders appear on a dedicated "Event reminders" channel; tapping one
+  opens the event's detail screen. Email reminders are never posted (the
+  provider only schedules alert-type reminders)
+- A one-time onboarding step after the calendar grant introduces reminders,
+  requests the notification permission (Android 13+), and warns that a second
+  calendar app with notifications on will duplicate them. "Not now" leaves
+  the feature off
+- Settings gained a "Notifications" section mirroring the choice: an event-
+  reminders toggle (default on) with the duplicate-reminders hint; turning it
+  on re-requests the notification permission when missing
+
 ## [1.3.0] — 2026-06-11
 
 ### Added