feat(reminders): snooze + dismiss notification actions

Add "Snooze" and "Dismiss" action buttons to reminder notifications.

The app is otherwise pure provider-broadcast (the Etar model): the calendar
provider fires EVENT_REMINDER and we post the notification, then mark the
CalendarAlerts row fired. A snoozed reminder has no provider backing — its
row is already fired — so snooze self-schedules an exact alarm to re-show
the same notification, while primary delivery is left unchanged.

- ReminderActionReceiver (not exported): SNOOZE cancels + schedules a
  re-show, DISMISS cancels, SHOW (the alarm) re-posts so it can be snoozed
  or dismissed again.
- ReminderSnoozeScheduler: setExactAndAllowWhileIdle, with an inexact
  allow-while-idle fallback if exact alarms are revoked (API 31-32).
- ReminderNotifier: two actions + cancel().
- snoozeMinutes pref (default 10) in Settings -> Notifications, OptionPicker
  presets 5/10/15/30/60.
- Manifest: USE_EXACT_ALARM + SCHEDULE_EXACT_ALARM (maxSdk 32) + receiver.
- New ic_notification_snooze/_dismiss drawables, duration plurals, en+de
  strings, snooze-pref tests.

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

app/src/main/AndroidManifest.xml

@@ -13,6 +13,17 @@
     -->
     <uses-permission android:name="android.permission.REQUEST_IGNORE_BATTERY_OPTIMIZATIONS" />
 
+    <!--
+      Re-fire a snoozed reminder at an exact time (the calendar provider won't —
+      its alert is already fired). USE_EXACT_ALARM is auto-granted to calendar
+      apps on API 33+; SCHEDULE_EXACT_ALARM covers API 31–32 (user-revocable,
+      with an inexact fallback if withheld). F-Droid-clean: no Play allowlisting.
+    -->
+    <uses-permission
+        android:name="android.permission.SCHEDULE_EXACT_ALARM"
+        android:maxSdkVersion="32" />
+    <uses-permission android:name="android.permission.USE_EXACT_ALARM" />
+
     <!-- Package visibility (Android 11+): without this, getLaunchIntentForPackage
          returns null and the calendar manager's per-account "manage" button can't
          open the source sync app (DAVx5, ICSx5, Google Calendar, …). The LAUNCHER
@@ -91,6 +102,13 @@
             </intent-filter>
         </receiver>
 
+        <!-- Snooze / dismiss actions on a reminder notification, plus the snooze
+             re-show alarm. Not exported: only our own notification buttons and
+             AlarmManager PendingIntents target it. -->
+        <receiver
+            android:name=".data.reminders.ReminderActionReceiver"
+            android:exported="false" />
+
         <!-- Home-screen widgets (Glance). Exported: the launcher/host binds them. -->
         <receiver
             android:name=".widget.agenda.AgendaWidgetReceiver"