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/db/enums.dart

@@ -101,3 +101,8 @@ enum PlantareDirection {
 /// Lifecycle of a Plantare. Framed as a promise, not a debt (data-model §2.7):
 /// `forgiven` (not "defaulted") when it's let go.
 enum PlantareStatus { open, returned, forgiven }
+
+/// Direction of a recorded seed Sale, seen from this app's owner. A sale is a
+/// SEPARATE model from a gift or a Plantare — seed for money (any currency:
+/// €, Ğ1, a local/time currency…). No commission is ever taken on seeds.
+enum SaleDirection { iSold, iBought }