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/lib/app.dart

@@ -27,6 +27,7 @@ import 'state/inventory_cubit.dart';
 import 'state/variety_detail_cubit.dart';
 import 'ui/about_screen.dart';
 import 'ui/auto_backup_gate.dart';
+import 'ui/calendar_screen.dart';
 import 'ui/chat_list_screen.dart';
 import 'ui/chat_screen.dart';
 import 'ui/home_screen.dart';
@@ -266,6 +267,12 @@ class TaneApp extends StatelessWidget {
           path: '/sales',
           builder: (context, state) => const SalesScreen(),
         ),
+        // "What's due this month" across the inventory — reads the per-variety
+        // crop calendar, no network.
+        GoRoute(
+          path: '/calendar',
+          builder: (context, state) => const CalendarScreen(),
+        ),
         GoRoute(
           path: '/variety/:id',
           builder: (context, state) => BlocProvider(