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/ui/variety_detail_screen.dart

@@ -18,6 +18,7 @@ import 'harvest_date_picker.dart';
 import 'photo_pick.dart';
 import 'plantare_sheet.dart';
 import 'quantity_kind_l10n.dart';
+import 'sale_sheet.dart';
 import 'quantity_picker.dart';
 import 'seed_glyph.dart';
 import 'theme.dart';
@@ -110,19 +111,31 @@ class _DetailView extends StatelessWidget {
             ],
           ),
           const SizedBox(height: 16),
-          _SectionTitle(t.plantare.title),
-          Align(
-            alignment: AlignmentDirectional.centerStart,
-            child: OutlinedButton.icon(
-              key: const Key('detail.addPlantare'),
-              icon: const Icon(Icons.volunteer_activism_outlined, size: 18),
-              onPressed: () => showPlantareSheet(
-                context,
-                repository: context.read<VarietyRepository>(),
-                varietyId: detail.id,
+          Wrap(
+            spacing: 8,
+            runSpacing: 8,
+            children: [
+              OutlinedButton.icon(
+                key: const Key('detail.addPlantare'),
+                icon: const Icon(Icons.volunteer_activism_outlined, size: 18),
+                onPressed: () => showPlantareSheet(
+                  context,
+                  repository: context.read<VarietyRepository>(),
+                  varietyId: detail.id,
+                ),
+                label: Text(t.plantare.add),
               ),
-              label: Text(t.plantare.add),
-            ),
+              OutlinedButton.icon(
+                key: const Key('detail.addSale'),
+                icon: const Icon(Icons.sell_outlined, size: 18),
+                onPressed: () => showSaleSheet(
+                  context,
+                  repository: context.read<VarietyRepository>(),
+                  varietyId: detail.id,
+                ),
+                label: Text(t.sale.add),
+              ),
+            ],
           ),
           if (detail.notes != null && detail.notes!.trim().isNotEmpty) ...[
             const SizedBox(height: 16),