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);
+    },
+  );
+}

apps/app_seeds/test/support/test_support.dart

@@ -8,6 +8,8 @@ import 'package:tane/data/variety_repository.dart';
 import 'package:tane/db/database.dart';
 import 'package:tane/i18n/strings.g.dart';
 import 'package:tane/state/inventory_cubit.dart';
+import 'package:tane/state/variety_detail_cubit.dart';
+import 'package:tane/ui/variety_detail_screen.dart';
 
 /// A fresh in-memory database for host tests (unencrypted; encryption is
 /// verified separately in the SQLCipher-only security test).
@@ -54,3 +56,30 @@ Widget wrapScreen({
     ),
   );
 }
+
+/// Wraps the [VarietyDetailScreen] for [varietyId] with its cubit + i18n.
+Widget wrapDetail({
+  required VarietyRepository repository,
+  required String varietyId,
+  AppLocale locale = AppLocale.en,
+}) {
+  LocaleSettings.setLocaleSync(locale);
+  return TranslationProvider(
+    child: RepositoryProvider.value(
+      value: repository,
+      child: BlocProvider(
+        create: (_) => VarietyDetailCubit(repository, varietyId),
+        child: MaterialApp(
+          locale: locale.flutterLocale,
+          supportedLocales: AppLocaleUtils.supportedLocales,
+          localizationsDelegates: const [
+            GlobalMaterialLocalizations.delegate,
+            GlobalWidgetsLocalizations.delegate,
+            GlobalCupertinoLocalizations.delegate,
+          ],
+          home: const VarietyDetailScreen(),
+        ),
+      ),
+    ),
+  );
+}

apps/app_seeds/test/ui/variety_detail_screen_test.dart

@@ -0,0 +1,92 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/db/database.dart';
+
+import '../support/test_support.dart';
+
+void main() {
+  late AppDatabase db;
+
+  setUp(() => db = newTestDatabase());
+  tearDown(() => db.close());
+
+  testWidgets('renders the variety label, category and its lots', (
+    tester,
+  ) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Maize', category: 'Poaceae');
+    await repo.addLot(
+      varietyId: id,
+      harvestYear: 2024,
+      quantity: const Quantity(kind: QuantityKind.cob),
+    );
+
+    await tester.pumpWidget(wrapDetail(repository: repo, varietyId: id));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Maize'), findsOneWidget); // app bar title
+    expect(find.text('Poaceae'), findsOneWidget); // category chip
+    expect(find.text('Year 2024 · a cob'), findsOneWidget); // lot line
+    await disposeTree(tester);
+  });
+
+  testWidgets('editing the name updates the title', (tester) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Old name');
+
+    await tester.pumpWidget(wrapDetail(repository: repo, varietyId: id));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('detail.edit')));
+    await tester.pumpAndSettle();
+
+    await tester.enterText(
+      find.byKey(const Key('editVariety.name')),
+      'New name',
+    );
+    await tester.tap(find.byKey(const Key('editVariety.save')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('New name'), findsOneWidget);
+    await disposeTree(tester);
+  });
+
+  testWidgets('adding a lot shows it in the list', (tester) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Bean');
+
+    await tester.pumpWidget(wrapDetail(repository: repo, varietyId: id));
+    await tester.pumpAndSettle();
+
+    expect(find.text('No lots yet.'), findsOneWidget);
+
+    await tester.tap(find.byKey(const Key('detail.addLot')));
+    await tester.pumpAndSettle();
+
+    await tester.enterText(find.byKey(const Key('addLot.year')), '2023');
+    await tester.tap(find.text('a handful'));
+    await tester.tap(find.byKey(const Key('addLot.save')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Year 2023 · a handful'), findsOneWidget);
+    await disposeTree(tester);
+  });
+
+  testWidgets('deleting the variety shows the not-found state', (tester) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Doomed');
+
+    await tester.pumpWidget(wrapDetail(repository: repo, varietyId: id));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('detail.delete')));
+    await tester.pumpAndSettle();
+    await tester.tap(find.byKey(const Key('detail.deleteConfirm')));
+    await tester.pumpAndSettle();
+
+    // The variety is soft-deleted: the reactive view re-emits null.
+    expect(find.text('This seed is no longer here.'), findsOneWidget);
+    await disposeTree(tester);
+  });
+}