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/test/db/schema/schema.dart

@@ -13,6 +13,7 @@ import 'schema_v6.dart' as v6;
 import 'schema_v7.dart' as v7;
 import 'schema_v8.dart' as v8;
 import 'schema_v9.dart' as v9;
+import 'schema_v10.dart' as v10;
 
 class GeneratedHelper implements SchemaInstantiationHelper {
   @override
@@ -36,10 +37,12 @@ class GeneratedHelper implements SchemaInstantiationHelper {
         return v8.DatabaseAtV8(db);
       case 9:
         return v9.DatabaseAtV9(db);
+      case 10:
+        return v10.DatabaseAtV10(db);
       default:
         throw MissingSchemaException(version, versions);
     }
   }
 
-  static const versions = const [1, 2, 3, 4, 5, 6, 7, 8, 9];
+  static const versions = const [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
 }