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/app.dart

@@ -33,6 +33,7 @@ import 'ui/home_screen.dart';
 import 'ui/intro_screen.dart';
 import 'ui/inventory_list_screen.dart';
 import 'ui/plantares_screen.dart';
+import 'ui/sales_screen.dart';
 import 'ui/market_offer_detail_screen.dart';
 import 'ui/market_screen.dart';
 import 'ui/offline_banner.dart';
@@ -260,6 +261,11 @@ class TaneApp extends StatelessWidget {
           path: '/plantares',
           builder: (context, state) => const PlantaresScreen(),
         ),
+        // Local sales ledger — also inventory-adjacent, no network.
+        GoRoute(
+          path: '/sales',
+          builder: (context, state) => const SalesScreen(),
+        ),
         GoRoute(
           path: '/variety/:id',
           builder: (context, state) => BlocProvider(