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/relative_date.dart

@@ -0,0 +1,18 @@
+/// Format a due date relative to today in German.
+///
+/// Returns labels like "Heute", "Morgen", "in X Tagen",
+/// "Uberfaellig seit 1 Tag", "Uberfaellig seit X Tagen".
+///
+/// Both [dueDate] and [today] are compared as date-only (ignoring time).
+String formatRelativeDate(DateTime dueDate, DateTime today) {
+  final diff =
+      DateTime(dueDate.year, dueDate.month, dueDate.day)
+          .difference(DateTime(today.year, today.month, today.day))
+          .inDays;
+
+  if (diff == 0) return 'Heute';
+  if (diff == 1) return 'Morgen';
+  if (diff > 1) return 'in $diff Tagen';
+  if (diff == -1) return 'Uberfaellig seit 1 Tag';
+  return 'Uberfaellig seit ${diff.abs()} Tagen';
+}