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/lib/app.dart

@@ -6,7 +6,9 @@ import 'package:go_router/go_router.dart';
 import 'data/variety_repository.dart';
 import 'i18n/strings.g.dart';
 import 'state/inventory_cubit.dart';
+import 'state/variety_detail_cubit.dart';
 import 'ui/inventory_list_screen.dart';
+import 'ui/variety_detail_screen.dart';
 
 /// Root widget. Provides the repository + inventory cubit to the tree and wires
 /// go_router. The list is `/`; `/variety/:id` is a placeholder detail route
@@ -30,8 +32,11 @@ class TaneApp extends StatelessWidget {
         ),
         GoRoute(
           path: '/variety/:id',
-          builder: (context, state) =>
-              _VarietyDetailPlaceholder(id: state.pathParameters['id']!),
+          builder: (context, state) => BlocProvider(
+            create: (_) =>
+                VarietyDetailCubit(repository, state.pathParameters['id']!),
+            child: const VarietyDetailScreen(),
+          ),
         ),
       ],
     );
@@ -60,17 +65,3 @@ class TaneApp extends StatelessWidget {
     );
   }
 }
-
-class _VarietyDetailPlaceholder extends StatelessWidget {
-  const _VarietyDetailPlaceholder({required this.id});
-
-  final String id;
-
-  @override
-  Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(),
-      body: Center(child: Text(id)),
-    );
-  }
-}