feat(todo): the Plan beside Upcoming, with what has no date (ADR 0033, D34)

Upcoming's 3-day grid gets the Plan Today's calendar has, through what
the two tabs now share (PlanHost). Its new last group holds the open
tasks of no day, so a week can be planned from the Inbox by dropping them
on an hour; they aren't counted on the button, which says what is due.

Whats-New: Upcoming's 3-day view has the Plan too, and tasks without a date can be dragged from it onto any day.
Whats-New-De: Auch die 3-Tage-Ansicht unter Demnächst hat den Plan, und Aufgaben ohne Datum lassen sich von dort auf jeden Tag ziehen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/test/feature/task/task_views_test.dart

@@ -135,6 +135,7 @@ void main() {
     expect(plan.toPlan.map((t) => t.id), ['open']);
     expect(plan.timed.map((t) => t.id), ['eight', 'nine']);
     expect(plan.count, 4);
+    expect(plan.noDate.map((t) => t.id), ['undated']);
 
     final home = planOf(tasks, today, projectIds: {'home'});
     expect(home.overdue, isEmpty);
@@ -142,4 +143,35 @@ void main() {
     expect(home.timed.map((t) => t.id), ['eight']);
     expect(home.count, 2);
   });
+
+  test('the Plan keeps the tasks of no day apart, uncounted (D34)', () {
+    const today = Day(2026, 11, 5);
+    OpenTask task(String id, {String? parentId, bool completed = false}) => (
+      id: id,
+      content: TaskContent(
+        title: id,
+        projectId: id == 'other' ? 'home' : 'inbox',
+        parentId: parentId,
+        completedAt: completed ? DateTime(2026, 11, 4) : null,
+      ),
+    );
+    final tasks = [
+      task('free'),
+      task('other'),
+      task('sub', parentId: 'free'),
+      task('done', completed: true),
+    ];
+
+    final plan = planOf(tasks, today);
+    expect([for (final task in plan.noDate) task.id], ['free', 'other']);
+    expect(plan.count, 0);
+    expect(plan.isEmpty, isFalse);
+    expect(
+      [
+        for (final task in planOf(tasks, today, projectIds: {'home'}).noDate)
+          task.id,
+      ],
+      ['other'],
+    );
+  });
 }