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/ui/app_drawer.dart

@@ -53,6 +53,15 @@ class AppDrawer extends StatelessWidget {
                 context.push('/sales');
               },
             ),
+            // "What's due this month" — the aggregate crop calendar, offline.
+            _DrawerItem(
+              icon: const Icon(Icons.calendar_month),
+              label: t.menu.calendar,
+              onTap: () {
+                Navigator.of(context).pop();
+                context.push('/calendar');
+              },
+            ),
             _DrawerItem(
               icon: const Icon(Symbols.storefront),
               label: t.menu.market,