feat(inventory): month/year harvest date picker (schema v3)

Replace the free-text harvest year with a themed month/year picker:
a tappable field opens a green-styled dialog with month and year grids
(navigable, paginated years), month optional. Lots now read e.g.
"September 2021" or just "Year 2021".

- schema v3: add nullable Lot.harvest_month (1..12) + migration + tests
- repo/cubit propagate harvestMonth; localized month names (en/es) via slang

apps/app_seeds/lib/db/tables.dart

@@ -46,6 +46,7 @@ class Lots extends Table with SyncColumns {
   TextColumn get type =>
       textEnum<LotType>().withDefault(const Constant('seed'))();
   IntColumn get harvestYear => integer().nullable()();
+  IntColumn get harvestMonth => integer().nullable()(); // 1..12, optional
   TextColumn get quantityKind => text().nullable()(); // QuantityKind.name
   RealColumn get quantityPrecise => real().nullable()();
   TextColumn get quantityLabel => text().nullable()();