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/ui/small_screen_overflow_test.dart

@@ -9,7 +9,10 @@ import 'package:tane/services/social_account_store.dart';
 import 'package:tane/services/social_connection.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
+import 'package:drift/drift.dart' show Value;
+import 'package:tane/domain/crop_calendar.dart';
 import 'package:tane/ui/about_screen.dart';
+import 'package:tane/ui/calendar_screen.dart';
 import 'package:tane/ui/chat_screen.dart';
 import 'package:tane/ui/home_screen.dart';
 import 'package:tane/ui/intro_screen.dart';
@@ -294,6 +297,24 @@ void main() {
     await disposeTree(tester);
   });
 
+  testWidgets('the "this month" calendar fits a small screen', (tester) async {
+    final db = newTestDatabase();
+    addTearDown(db.close);
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(
+        label: 'Lechuga maravilla de verano', category: 'Asteráceas');
+    await repo.updateVariety(id: id, sowMonths: Value(monthsToMask([3])));
+    await pumpSmall(
+      tester,
+      wrapScreen(
+        repository: repo,
+        locale: AppLocale.es,
+        child: const CalendarScreen(initialMonth: 3),
+      ),
+    );
+    await disposeTree(tester);
+  });
+
   // NOTE: the variety-detail screen is deliberately NOT overflow-checked here.
   // It reports a ~2px overflow on a RenderFlex that is already DISPOSED/DEFUNCT
   // — a transient stale frame while its cubit's Drift stream rebuilds during