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/database.dart

@@ -27,7 +27,7 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase(super.e);
 
   @override
-  int get schemaVersion => 2;
+  int get schemaVersion => 3;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -37,6 +37,10 @@ class AppDatabase extends _$AppDatabase {
       if (from < 2) {
         await m.addColumn(lots, lots.type);
       }
+      // v3: optional harvest month alongside the harvest year.
+      if (from < 3) {
+        await m.addColumn(lots, lots.harvestMonth);
+      }
     },
   );
 }