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/i18n/es.i18n.json

@@ -56,10 +56,19 @@
   },
   "addLot": {
     "title": "Añadir lote",
-    "year": "Año de cosecha",
+    "year": "Fecha de cosecha",
     "quantity": "¿Cuánta?",
     "amount": "Cantidad"
   },
+  "harvest": {
+    "pickTitle": "Seleccionar mes / año",
+    "anyMonth": "Cualquier mes",
+    "noDate": "Indicar fecha de cosecha",
+    "monthNames": [
+      "enero", "febrero", "marzo", "abril", "mayo", "junio",
+      "julio", "agosto", "septiembre", "octubre", "noviembre", "diciembre"
+    ]
+  },
   "lotType": {
     "seed": "Semillas",
     "plant": "Plantel"