feat(11-01): recalculate nextDueDate from today on non-due-day completion

- When completing on due date: use original due date as base (preserves rhythm)
- When completing on different day: use today as base (per D-02)
- Replace todayDateOnly with todayStart used for both base calculation and catch-up
- Update doc comment to reflect new behavior

lib/features/tasks/data/tasks_dao.dart

@@ -35,8 +35,10 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
 
   /// Mark a task as done: records completion and calculates next due date.
   ///
-  /// Uses scheduling utility for date calculation. Next due is calculated
-  /// from the original due date (not completion date) to keep rhythm stable.
+  /// Uses scheduling utility for date calculation. If completing on the due
+  /// date, next due is calculated from the original due date (keeps rhythm).
+  /// If completing on a different day (early or late), next due is calculated
+  /// from today (per D-02: matches user mental model "I did it now, schedule next from now").
   /// If the calculated next due is in the past, catch-up advances to present.
   ///
   /// [now] parameter allows injection of current time for testing.
@@ -54,23 +56,24 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
         completedAt: currentTime,
       ));
 
-      // 3. Calculate next due date (from original due date, not today)
+      // 3. Calculate next due date
+      // If completing on the due date, use original due date as base (keeps rhythm).
+      // If completing on a different day (early or late), use today as base (per D-02).
+      final todayStart = DateTime(currentTime.year, currentTime.month, currentTime.day);
+      final taskDueDay = DateTime(task.nextDueDate.year, task.nextDueDate.month, task.nextDueDate.day);
+      final baseDate = todayStart == taskDueDay ? task.nextDueDate : todayStart;
+
       var nextDue = calculateNextDueDate(
-        currentDueDate: task.nextDueDate,
+        currentDueDate: baseDate,
         intervalType: task.intervalType,
         intervalDays: task.intervalDays,
         anchorDay: task.anchorDay,
       );
 
       // 4. Catch up if next due is still in the past
-      final todayDateOnly = DateTime(
-        currentTime.year,
-        currentTime.month,
-        currentTime.day,
-      );
       nextDue = catchUpToPresent(
         nextDue: nextDue,
-        today: todayDateOnly,
+        today: todayStart,
         intervalType: task.intervalType,
         intervalDays: task.intervalDays,
         anchorDay: task.anchorDay,