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/en.i18n.json

@@ -56,10 +56,19 @@
   },
   "addLot": {
     "title": "Add lot",
-    "year": "Harvest year",
+    "year": "Harvest date",
     "quantity": "How much?",
     "amount": "Amount"
   },
+  "harvest": {
+    "pickTitle": "Select month / year",
+    "anyMonth": "Any month",
+    "noDate": "Set harvest date",
+    "monthNames": [
+      "January", "February", "March", "April", "May", "June",
+      "July", "August", "September", "October", "November", "December"
+    ]
+  },
   "lotType": {
     "seed": "Seeds",
     "plant": "Plants"