feat(02-01): Drift tables, DAOs, scheduling utility, domain models with tests

- Add Rooms, Tasks, TaskCompletions Drift tables with schema v2 migration
- Create RoomsDao with CRUD, watchAll, watchWithStats, cascade delete, reorder
- Create TasksDao with CRUD, watchInRoom (sorted by due), completeTask, overdue detection
- Implement calculateNextDueDate and catchUpToPresent pure scheduling functions
- Define IntervalType enum (8 types), EffortLevel enum, FrequencyInterval model
- Add formatRelativeDate German formatter and curatedRoomIcons icon list
- Enable PRAGMA foreign_keys in beforeOpen migration strategy
- All 30 unit tests passing (17 scheduling + 6 rooms DAO + 7 tasks DAO)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/features/tasks/domain/scheduling.dart

@@ -0,0 +1,66 @@
+import 'frequency.dart';
+
+/// Calculate the next due date from the current due date and interval config.
+///
+/// For calendar-anchored intervals, [anchorDay] is the original day-of-month.
+/// Day-count intervals use pure arithmetic (Duration).
+/// Calendar-anchored intervals use month arithmetic with clamping.
+DateTime calculateNextDueDate({
+  required DateTime currentDueDate,
+  required IntervalType intervalType,
+  required int intervalDays,
+  int? anchorDay,
+}) {
+  switch (intervalType) {
+    // Day-count: pure arithmetic
+    case IntervalType.daily:
+      return currentDueDate.add(const Duration(days: 1));
+    case IntervalType.everyNDays:
+      return currentDueDate.add(Duration(days: intervalDays));
+    case IntervalType.weekly:
+      return currentDueDate.add(const Duration(days: 7));
+    case IntervalType.biweekly:
+      return currentDueDate.add(const Duration(days: 14));
+    // Calendar-anchored: month arithmetic with clamping
+    case IntervalType.monthly:
+      return _addMonths(currentDueDate, 1, anchorDay);
+    case IntervalType.everyNMonths:
+      return _addMonths(currentDueDate, intervalDays, anchorDay);
+    case IntervalType.quarterly:
+      return _addMonths(currentDueDate, 3, anchorDay);
+    case IntervalType.yearly:
+      return _addMonths(currentDueDate, 12, anchorDay);
+  }
+}
+
+/// Add months with day-of-month clamping.
+/// [anchorDay] remembers the original day for correct clamping.
+DateTime _addMonths(DateTime date, int months, int? anchorDay) {
+  final targetMonth = date.month + months;
+  final targetYear = date.year + (targetMonth - 1) ~/ 12;
+  final normalizedMonth = ((targetMonth - 1) % 12) + 1;
+  final day = anchorDay ?? date.day;
+  // Last day of target month: day 0 of next month
+  final lastDay = DateTime(targetYear, normalizedMonth + 1, 0).day;
+  final clampedDay = day > lastDay ? lastDay : day;
+  return DateTime(targetYear, normalizedMonth, clampedDay);
+}
+
+/// Catch-up: if next due is in the past, keep adding intervals until future/today.
+DateTime catchUpToPresent({
+  required DateTime nextDue,
+  required DateTime today,
+  required IntervalType intervalType,
+  required int intervalDays,
+  int? anchorDay,
+}) {
+  while (nextDue.isBefore(today)) {
+    nextDue = calculateNextDueDate(
+      currentDueDate: nextDue,
+      intervalType: intervalType,
+      intervalDays: intervalDays,
+      anchorDay: anchorDay,
+    );
+  }
+  return nextDue;
+}