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>

app/src/main/java/de/jeanlucmakiola/calendula/ui/CalendarHost.kt

@@ -8,6 +8,7 @@ import androidx.compose.animation.slideOutHorizontally
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -29,9 +30,18 @@ import kotlinx.datetime.LocalDate
  * Holds the active top-level view (spec M1) and swaps between the calendar
  * screens. Each screen owns its own ViewModel and date anchor; the view-switcher
  * pill in their top bars writes back here via [onSelectView].
+ *
+ * [requestedDetailKey] is an externally requested occurrence (a tapped
+ * reminder notification routed through MainActivity): it opens the detail
+ * overlay exactly like an event tap and is cleared via [onDetailKeyConsumed]
+ * so a later recomposition can't re-open it.
  */
 @Composable
-fun CalendarHost(modifier: Modifier = Modifier) {
+fun CalendarHost(
+    modifier: Modifier = Modifier,
+    requestedDetailKey: LongArray? = null,
+    onDetailKeyConsumed: () -> Unit = {},
+) {
     var view by rememberSaveable { mutableStateOf(CalendarView.Week) }
     val onSelectView: (CalendarView) -> Unit = { view = it }
 
@@ -61,6 +71,15 @@ fun CalendarHost(modifier: Modifier = Modifier) {
         detailKey = key
     }
 
+    // A tapped reminder notification asks for a specific occurrence.
+    LaunchedEffect(requestedDetailKey) {
+        if (requestedDetailKey != null) {
+            heldKey = requestedDetailKey
+            detailKey = requestedDetailKey
+            onDetailKeyConsumed()
+        }
+    }
+
     // Settings (M4) is hoisted here so it overlays whichever calendar view is
     // active and survives view switches. (The calendar filter now lives inline
     // in the navigation drawer, so no overlay state is needed for it.)