feat(sales): local sales/purchase ledger (any currency)

A sale is a distinct model from a gift or a Plantare (reproduction
commitment): a recorded seed sale or purchase with an optional price in
ANY currency — €, Ğ1, time, or none yet. Mirrors the Plantare feature.

- schema v10: Sales table (SyncColumns), guarded createTable migration,
  schema dump + generated schema_v10 for the migration round-trip test
- enum SaleDirection { iSold, iBought }
- VarietyRepository: create/watch/watchForVariety/delete + backup
  export/exportForSync/import (LWW-by-HLC, tombstones)
- InventorySnapshot.sales + JSON codec encode/decode (round-trips amount,
  currency, counterparty)
- UI: SalesScreen (/sales), sale sheet, drawer entry, variety-detail
  action beside 'add Plantare'; money hides a trailing .0
- i18n sale block + menu.sales in en/es/pt/ast
- tests: sales repo test (5) incl. Ğ1/price-less/backup round-trip;
  Sales screen added to the small-screen overflow guard

Also harden the overflow guard: swallow the headless engine's image
resource service PNG-decode errors (unrelated to layout) while still
failing on real RenderFlex overflows, so home/about stop reporting
false failures.

apps/app_seeds/lib/i18n/pt.i18n.json

@@ -38,6 +38,7 @@
     "wishlist": "Lista de desejos",
     "following": "A seguir",
     "plantares": "Plantares",
+    "sales": "Vendas",
     "settings": "Definições"
   },
   "settings": {
@@ -576,5 +577,23 @@
     "openSection": "Pendentes",
     "settledSection": "Feitos",
     "removeConfirm": "Remover este compromisso?"
+  },
+  "sale": {
+    "title": "Vendas",
+    "help": "Regista semente vendida ou comprada — dinheiro, Ğ1 ou qualquer moeda. Um modelo separado do presente e do Plantare. Nunca se cobra comissão pelas sementes.",
+    "add": "Registar venda",
+    "empty": "Ainda não há vendas. Anota aqui o que vendes ou compras.",
+    "iSold": "Vendi",
+    "iBought": "Comprei",
+    "direction": "Vendes ou compras?",
+    "counterparty": "Com quem?",
+    "counterpartyHint": "Uma pessoa ou um coletivo (opcional)",
+    "amount": "Montante",
+    "currency": "Moeda",
+    "currencyHint": "€, Ğ1, horas… (opcional)",
+    "note": "Nota (opcional)",
+    "save": "Guardar",
+    "delete": "Remover",
+    "removeConfirm": "Remover esta venda?"
   }
 }