feat(11-02): render pre-populated tasks with muted visual distinction in calendar UI

- Add isPrePopulated parameter to CalendarTaskRow (default false, backward compat)
- Wrap ListTile in Opacity(0.55) when isPrePopulated to indicate upcoming tasks
- Render 'Demnächst' section in CalendarDayList with muted section header
- Update _buildAnimatedTaskRow to accept and forward isPrePopulated flag
- Update _CompletingTaskRow to accept isPrePopulated (always false during animation)
- Update celebration check to include prePopulatedTasks.isEmpty condition
- Update _completingTaskIds cleanup to also check prePopulatedTasks

lib/features/home/presentation/calendar_task_row.dart

@@ -14,6 +14,10 @@ const _overdueColor = Color(0xFFE07A5F);
 ///
 /// When [isOverdue] is true the task name uses coral text to visually
 /// distinguish overdue carry-over from today's regular tasks.
+///
+/// When [isPrePopulated] is true the entire row is rendered at 0.55 opacity
+/// to indicate it is not yet due (visible within interval window, but due
+/// date is in the future).
 class CalendarTaskRow extends StatelessWidget {
   const CalendarTaskRow({
     super.key,
@@ -22,6 +26,7 @@ class CalendarTaskRow extends StatelessWidget {
     this.isOverdue = false,
     this.showRoomTag = true,
     this.canComplete = true,
+    this.isPrePopulated = false,
   });
 
   final TaskWithRoom taskWithRoom;
@@ -38,12 +43,16 @@ class CalendarTaskRow extends StatelessWidget {
   /// When false, the checkbox is disabled (e.g. for future tasks).
   final bool canComplete;
 
+  /// When true, the row is rendered at 0.55 opacity to indicate an
+  /// upcoming (not-yet-due) pre-populated task within its interval window.
+  final bool isPrePopulated;
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
     final task = taskWithRoom.task;
 
-    return ListTile(
+    final tile = ListTile(
       onTap: () => context.go(
         '/rooms/${taskWithRoom.roomId}/tasks/${taskWithRoom.task.id}',
       ),
@@ -79,5 +88,7 @@ class CalendarTaskRow extends StatelessWidget {
             )
           : null,
     );
+
+    return isPrePopulated ? Opacity(opacity: 0.55, child: tile) : tile;
   }
 }