feat(todo): the Inbox's Calendar as an Agenda or 3-day

As Todoist's Display in the Inbox: once its layout is Calendar, a row
"Layout: Agenda" opens a small choice of Agenda or 3-day; 3-day shows the
Inbox's tasks in Upcoming's HourGrid, whatever the window's width, where a
tap on an hour adds to the Inbox. Kept as the project's other Display
choices, in its sealed view (`calendarLayout`, written only when it isn't
the Agenda; a value this version doesn't know reads as the Agenda).
Another project's Calendar stays the agenda (or the month), as Todoist
offers the choice in the Inbox alone.

Whats-New: The Inbox's Calendar can show three days of hours: Display, Layout, 3-day.
Whats-New-De: Der Kalender des Eingangs kann drei Tage mit ihren Stunden zeigen: Anzeige, Layout, 3 Tage.
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/project/project_payload_test.dart

@@ -61,6 +61,30 @@ void main() {
     });
   });
 
+  test(
+    "the Inbox's Calendar keeps its layout, Agenda or 3-day, written "
+    "only where it isn't the Agenda (the developer's review, 7 Nov 2026)",
+    () {
+      const threeDay = ProjectView(
+        layout: ProjectLayout.calendar,
+        calendarLayout: CalendarLayout.threeDay,
+      );
+      expect(encodeView(threeDay), {
+        'layout': 'calendar',
+        'calendarLayout': 'threeDay',
+      });
+      expect(decodeView(encodeView(threeDay)), threeDay);
+      expect(encodeView(const ProjectView(layout: ProjectLayout.calendar)), {
+        'layout': 'calendar',
+      });
+      expect(
+        decodeView({'layout': 'calendar', 'calendarLayout': 'week'}),
+        const ProjectView(layout: ProjectLayout.calendar),
+        reason: 'a layout this version does not know: the Agenda',
+      );
+    },
+  );
+
   test('a Display value this version does not know reads as at first', () {
     final read = decodeProject(
       json({