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/test/db/schema/schema.dart

@@ -6,6 +6,7 @@ import 'package:drift/drift.dart';
 import 'package:drift/internal/migrations.dart';
 import 'schema_v1.dart' as v1;
 import 'schema_v2.dart' as v2;
+import 'schema_v3.dart' as v3;
 
 class GeneratedHelper implements SchemaInstantiationHelper {
   @override
@@ -15,10 +16,12 @@ class GeneratedHelper implements SchemaInstantiationHelper {
         return v1.DatabaseAtV1(db);
       case 2:
         return v2.DatabaseAtV2(db);
+      case 3:
+        return v3.DatabaseAtV3(db);
       default:
         throw MissingSchemaException(version, versions);
     }
   }
 
-  static const versions = const [1, 2];
+  static const versions = const [1, 2, 3];
 }