feat(todo): a project's Calendar as a month on wide windows (ADR 0033, D17)

From 720 pixels of the Calendar's own width on, a month grid in place
of the agenda: the month's weeks from the week's first day, the days
of the months beside it muted, each day with as many of its tasks as
fit and the rest counted, opening the day's tasks in a sheet; past
days with their overdue and done tasks. A task held and dropped on
another day moves there (moveTask), a day's free space tapped adds a
task on it, a line tapped opens its task.

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_calendar_test.dart

@@ -171,4 +171,89 @@ void main() {
     );
     expect(shown(calendar), {today: 'a,b'});
   });
+
+  group('a month as a grid (D17)', () {
+    test('its weeks from the one with its first day to the one with its '
+        'last, starting on the week’s first day', () {
+      // November 2026 starts on a Sunday and ends on a Monday.
+      expect(monthWeeks(const Day(2026, 11, 1), DateTime.monday), [
+        const Day(2026, 10, 26),
+        const Day(2026, 11, 2),
+        const Day(2026, 11, 9),
+        const Day(2026, 11, 16),
+        const Day(2026, 11, 23),
+        const Day(2026, 11, 30),
+      ]);
+      expect(monthWeeks(const Day(2026, 11, 20), DateTime.sunday), [
+        const Day(2026, 11, 1),
+        const Day(2026, 11, 8),
+        const Day(2026, 11, 15),
+        const Day(2026, 11, 22),
+        const Day(2026, 11, 29),
+      ]);
+      // February 2027 fills four weeks from a Monday.
+      expect(monthWeeks(const Day(2027, 2, 1), DateTime.monday), hasLength(4));
+    });
+
+    test('each day from its first to before its end with its tasks, past '
+        'ones too, done ones where kept, by time', () {
+      final days = monthOf(
+        [
+          task('none'),
+          task('late', day: today.plus(-3)),
+          task('done', day: today.plus(-1), completed: true),
+          task('noon', day: today.plus(2), time: 12 * 60),
+          task('morning', day: today.plus(2), time: 8 * 60),
+          task('after', day: today.plus(30)),
+        ],
+        today,
+        from: today.plus(-5),
+        to: today.plus(30),
+      );
+      expect(days.keys.first, today.plus(-5));
+      expect(days, hasLength(35));
+      expect(
+        {
+          for (final MapEntry(:key, :value) in days.entries)
+            if (value.isNotEmpty)
+              key: [for (final entry in value) entry.task.id].join(','),
+        },
+        {
+          today.plus(-3): 'late',
+          today.plus(-1): 'done',
+          today.plus(2): 'morning,noon',
+        },
+      );
+    });
+
+    test('later times of repeating tasks from today on alone, where asked', () {
+      final weekly = task(
+        'weekly',
+        day: today.plus(-7),
+        repeat: const Repeat(RepeatUnit.week),
+      );
+      expect(
+        monthOf([weekly], today, from: today.plus(-14), to: today.plus(14))
+            .values
+            .expand((entries) => entries)
+            .map((entry) => entry.task.content.date!.day),
+        [today.plus(-7)],
+      );
+      final days = monthOf(
+        [weekly],
+        today,
+        from: today.plus(-14),
+        to: today.plus(14),
+        occurrences: true,
+      );
+      final later = [
+        for (final MapEntry(:key, :value) in days.entries)
+          for (final entry in value)
+            if (entry.occurrence) key,
+      ];
+      // Overdue a week, done today it would come a week on, then the
+      // grid ends.
+      expect(later, [today.plus(7)]);
+    });
+  });
 }