feat(calendar): "what's due this month" — dedicated screen + inventory filter

Reuses the existing per-variety crop-calendar masks (sow/transplant/
flowering/fruiting/seed-harvest) — no schema change, no migration — to
answer "qué toca este mes" across the whole inventory. Since the months
are the grower's own record, there's no hemisphere assumption.

- Repo: CalendarEntry + watchCalendar() (varieties with any recorded
  phase); VarietyListItem now carries sowMonths.
- Dedicated CalendarScreen (/calendar): a month strip (defaults to the
  current month, intl month names) and varieties grouped by phase —
  actions (sow/transplant/harvest seed) above informational (flowering/
  fruiting), each tinted, tap → variety detail; empty-month state.
- Inventory filter: a 'this month' chip (shown only when some variety
  has a calendar) keeps what sows in the current month; folded into the
  attributes group + clear-filters.
- Entry points: a live Home card + a drawer item.
- i18n calendar block + menu.calendar (en/es/pt/ast).
- Tests: calendar_test (watchCalendar, list carries sow-months, pure
  filter), calendar_screen_test (phase grouping, month switch, empty),
  plus the screen added to the small-screen overflow guard.

apps/app_seeds/test/data/calendar_test.dart

@@ -0,0 +1,65 @@
+import 'package:drift/drift.dart' show Value;
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/data/variety_repository.dart';
+import 'package:tane/db/database.dart';
+import 'package:tane/domain/crop_calendar.dart';
+import 'package:tane/state/inventory_cubit.dart';
+
+import '../support/test_support.dart';
+
+/// The aggregate crop calendar: `watchCalendar` surfaces every variety with a
+/// recorded phase, and the inventory carries sow-months for the "this month"
+/// filter.
+void main() {
+  late AppDatabase db;
+  setUp(() => db = newTestDatabase());
+  tearDown(() => db.close());
+
+  test('watchCalendar emits varieties with a recorded phase, masks intact',
+      () async {
+    final repo = newTestRepository(db);
+    final tomato = await repo.addQuickVariety(label: 'Tomate rosa');
+    await repo.updateVariety(
+      id: tomato,
+      sowMonths: Value(monthsToMask([2, 3])), // Feb, Mar
+      seedHarvestMonths: Value(monthsToMask([8])), // Aug
+    );
+    // A variety with no calendar at all is excluded.
+    await repo.addQuickVariety(label: 'Sin calendario');
+
+    final entries = await repo.watchCalendar().first;
+    expect(entries, hasLength(1));
+    final e = entries.single;
+    expect(e.label, 'Tomate rosa');
+    expect(maskHasMonth(e.sowMonths, 3), isTrue);
+    expect(maskHasMonth(e.sowMonths, 5), isFalse);
+    expect(maskHasMonth(e.seedHarvestMonths, 8), isTrue);
+  });
+
+  test('the inventory list carries sow-months', () async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Lechuga');
+    await repo.updateVariety(id: id, sowMonths: Value(monthsToMask([9])));
+
+    final items = await repo.watchInventory().first;
+    expect(items.single.sowMonths, monthsToMask([9]));
+  });
+
+  test('the sow-this-month filter keeps only what sows in the pinned month', () {
+    const march = 3;
+    final sowsInMarch = _item('a', monthsToMask([3, 4]));
+    final sowsInJune = _item('b', monthsToMask([6]));
+    final noCalendar = _item('c', null);
+    final state = InventoryState(
+      items: [sowsInMarch, sowsInJune, noCalendar],
+      sowThisMonthOnly: true,
+      filterMonth: march,
+      loading: false,
+    );
+    expect(state.visibleItems.map((i) => i.id), ['a']);
+    expect(state.hasSowCalendar, isTrue);
+  });
+}
+
+VarietyListItem _item(String id, int? sowMonths) =>
+    VarietyListItem(id: id, label: id, sowMonths: sowMonths);