feat: persist timetable and validate confirmed imports

mobile/lib/services/calendar_service.dart

@@ -0,0 +1,17 @@
+import '../domain/course_occurrence.dart';
+
+class DeviceCalendar {
+  const DeviceCalendar(this.id, this.name);
+  final String id, name;
+}
+
+abstract class CalendarService {
+  Future<bool> requestAccess();
+  Future<List<DeviceCalendar>> listWritableCalendars();
+  Future<Map<String, String>> sync(
+    List<CourseOccurrence> occurrences,
+    String calendarId,
+    Map<String, String> knownEventIds,
+  );
+  Future<void> deleteOwned(Map<String, String> eventIds);
+}

mobile/lib/services/notification_service.dart

@@ -0,0 +1,25 @@
+import '../domain/app_settings.dart';
+import '../domain/course_occurrence.dart';
+
+enum ReminderStatus { off, permissionDenied, exact, inexact, error }
+
+class ReminderResult {
+  const ReminderResult(
+    this.status, {
+    this.scheduledCount = 0,
+    this.message = '',
+  });
+  final ReminderStatus status;
+  final int scheduledCount;
+  final String message;
+}
+
+abstract class NotificationService {
+  Future<ReminderResult> reconcile(
+    List<CourseOccurrence> occurrences,
+    AppSettings settings,
+    DateTime now,
+  );
+  Future<void> requestPermissions();
+  Future<void> cancelOwned();
+}