feat(block1): variety detail + edit screen with reactive lots

Tapping an inventory item now opens its detail (the /variety/:id route was a
placeholder). Read + edit one variety end-to-end:

- Detail view: photo, category, "also known as" names, notes and lots, driven
  by a reactive VarietyDetailCubit.
- Edit core fields (label/category/notes, LWW) via a bottom sheet.
- Add a lot (harvest year + quantity) — the list refreshes reactively.
- Soft-delete (tombstone) with a confirm dialog.

Repository:
- watchVariety(id): merges Drift table-change streams (variety, lots, names,
  attachments) via StreamGroup and reloads the full detail on any change, so a
  lot added to a *different* table still refreshes the view.
- updateVariety (LWW), addLot, softDeleteVariety.

i18n: detail/edit/addLot strings (ES/EN); switched slang to {param} braces
interpolation (translator- and Weblate-friendly).

Tests: repository (reactive watchVariety, update, soft-delete) and widget tests
(render, edit updates title, add-lot appears, delete shows not-found). Full
suite green: 23 passing, 1 skipped (SQLCipher-only encryption guard).

apps/app_seeds/test/data/variety_detail_test.dart

@@ -0,0 +1,63 @@
+import 'package:async/async.dart';
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/data/variety_repository.dart';
+import 'package:tane/db/database.dart';
+
+import '../support/test_support.dart';
+
+void main() {
+  late AppDatabase db;
+  late VarietyRepository repo;
+
+  setUp(() {
+    db = newTestDatabase();
+    repo = newTestRepository(db);
+  });
+  tearDown(() => db.close());
+
+  test('watchVariety emits null for a missing variety', () async {
+    expect(await repo.watchVariety('does-not-exist').first, isNull);
+  });
+
+  test('watchVariety reacts to a lot being added', () async {
+    final id = await repo.addQuickVariety(label: 'Maize', category: 'Poaceae');
+    final queue = StreamQueue(repo.watchVariety(id));
+
+    final initial = await queue.next;
+    expect(initial!.label, 'Maize');
+    expect(initial.lots, isEmpty);
+
+    await repo.addLot(
+      varietyId: id,
+      harvestYear: 2024,
+      quantity: const Quantity(kind: QuantityKind.cob),
+    );
+
+    final updated = await queue.next;
+    expect(updated!.lots.single.harvestYear, 2024);
+    expect(updated.lots.single.quantity?.kind, QuantityKind.cob);
+
+    await queue.cancel();
+  });
+
+  test('updateVariety changes scalar fields (last-writer-wins)', () async {
+    final id = await repo.addQuickVariety(label: 'Old name');
+    await repo.updateVariety(id: id, label: 'New name', notes: 'a note');
+
+    final detail = await repo.watchVariety(id).first;
+    expect(detail!.label, 'New name');
+    expect(detail.notes, 'a note');
+  });
+
+  test(
+    'softDeleteVariety hides it from watchVariety and the inventory',
+    () async {
+      final id = await repo.addQuickVariety(label: 'Doomed');
+      await repo.softDeleteVariety(id);
+
+      expect(await repo.watchVariety(id).first, isNull);
+      expect(await repo.watchInventory().first, isEmpty);
+    },
+  );
+}