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/database.dart

@@ -23,6 +23,7 @@ part 'database.g.dart';
     Attachments,
     ExternalLinks,
     Plantares,
+    Sales,
   ],
 )
 class AppDatabase extends _$AppDatabase {
@@ -30,7 +31,7 @@ class AppDatabase extends _$AppDatabase {
 
   /// Current schema version; also stamped into interchange exports so an
   /// importer knows which app generation wrote the file (data-model §7).
-  static const int currentSchemaVersion = 9;
+  static const int currentSchemaVersion = 10;
 
   @override
   int get schemaVersion => currentSchemaVersion;
@@ -146,6 +147,12 @@ class AppDatabase extends _$AppDatabase {
           await m.createTable(plantares);
         }
       }
+      // v10: Sales — recorded seed sales (separate from gift/Plantare). Guarded.
+      if (from < 10) {
+        if (!await _hasTable('sales')) {
+          await m.createTable(sales);
+        }
+      }
     },
   );