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/state/inventory_cubit.dart

@@ -5,6 +5,7 @@ import 'package:flutter_bloc/flutter_bloc.dart';
 
 import '../data/variety_repository.dart';
 import '../db/enums.dart';
+import '../domain/crop_calendar.dart';
 
 /// Inventory list state: all items from the DB plus the current search query
 /// and active filters. [visibleItems] applies query ∧ category ∧ form; grouping
@@ -19,6 +20,8 @@ class InventoryState extends Equatable {
     this.organicOnly = false,
     this.needsReproductionOnly = false,
     this.sharingOnly = false,
+    this.sowThisMonthOnly = false,
+    this.filterMonth = 0,
     this.loading = true,
     this.selectionMode = false,
     this.selectedIds = const {},
@@ -49,6 +52,15 @@ class InventoryState extends Equatable {
   /// "what I share" view.
   final bool sharingOnly;
 
+  /// When true, keep only varieties whose recorded sow-months include
+  /// [filterMonth] — the inventory "to sow this month" view.
+  final bool sowThisMonthOnly;
+
+  /// The month (1..12) the "to sow this month" filter matches against — set to
+  /// the current month by the cubit when the filter is turned on. Only read
+  /// while [sowThisMonthOnly] is true.
+  final int filterMonth;
+
   final bool loading;
 
   /// Whether the list is in multi-select mode — used to pick a subset of the
@@ -84,10 +96,17 @@ class InventoryState extends Equatable {
       if (organicOnly && !i.isOrganic) return false;
       if (needsReproductionOnly && !i.needsReproduction) return false;
       if (sharingOnly && !i.isShared) return false;
+      if (sowThisMonthOnly && !maskHasMonth(i.sowMonths, filterMonth)) {
+        return false;
+      }
       return true;
     }).toList();
   }
 
+  /// Whether any variety has recorded sow-months, so the UI only offers the
+  /// "to sow this month" chip when it would match something.
+  bool get hasSowCalendar => items.any((i) => i.sowMonths != null);
+
   /// Whether any item is flagged organic, so the UI only offers the eco filter
   /// chip when it would actually match something.
   bool get hasOrganic => items.any((i) => i.isOrganic);
@@ -109,6 +128,8 @@ class InventoryState extends Equatable {
     bool? organicOnly,
     bool? needsReproductionOnly,
     bool? sharingOnly,
+    bool? sowThisMonthOnly,
+    int? filterMonth,
     bool? loading,
     bool? selectionMode,
     Set<String>? selectedIds,
@@ -123,6 +144,8 @@ class InventoryState extends Equatable {
       needsReproductionOnly:
           needsReproductionOnly ?? this.needsReproductionOnly,
       sharingOnly: sharingOnly ?? this.sharingOnly,
+      sowThisMonthOnly: sowThisMonthOnly ?? this.sowThisMonthOnly,
+      filterMonth: filterMonth ?? this.filterMonth,
       loading: loading ?? this.loading,
       selectionMode: selectionMode ?? this.selectionMode,
       selectedIds: selectedIds ?? this.selectedIds,
@@ -139,6 +162,8 @@ class InventoryState extends Equatable {
     organicOnly,
     needsReproductionOnly,
     sharingOnly,
+    sowThisMonthOnly,
+    filterMonth,
     loading,
     selectionMode,
     selectedIds,
@@ -148,7 +173,9 @@ class InventoryState extends Equatable {
 /// Subscribes to the repository's reactive inventory stream. The list updates
 /// automatically after a quick-add — no manual refresh.
 class InventoryCubit extends Cubit<InventoryState> {
-  InventoryCubit(this._repo) : super(const InventoryState()) {
+  InventoryCubit(this._repo, {int Function()? nowMonth})
+      : _nowMonth = nowMonth ?? (() => DateTime.now().month),
+        super(const InventoryState()) {
     // One combined subscription (list + draft tray). Two separate StreamGroups
     // here re-emit in a loop that hangs widget tests — see watchInventoryView.
     _sub = _repo.watchInventoryView().listen(
@@ -159,6 +186,7 @@ class InventoryCubit extends Cubit<InventoryState> {
   }
 
   final VarietyRepository _repo;
+  final int Function() _nowMonth;
   late final StreamSubscription<
     ({List<VarietyListItem> items, List<VarietyListItem> drafts})
   >
@@ -192,6 +220,12 @@ class InventoryCubit extends Cubit<InventoryState> {
   void toggleSharingOnly() =>
       emit(state.copyWith(sharingOnly: !state.sharingOnly));
 
+  /// Toggles the "to sow this month" filter, pinned to the current month.
+  void toggleSowThisMonth() => emit(state.copyWith(
+        sowThisMonthOnly: !state.sowThisMonthOnly,
+        filterMonth: _nowMonth(),
+      ));
+
   /// Enters multi-select mode with an empty selection (from the toolbar).
   void startSelection() =>
       emit(state.copyWith(selectionMode: true, selectedIds: const {}));
@@ -230,6 +264,7 @@ class InventoryCubit extends Cubit<InventoryState> {
       organicOnly: false,
       needsReproductionOnly: false,
       sharingOnly: false,
+      sowThisMonthOnly: false,
     ),
   );