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/slang.yaml

@@ -7,6 +7,9 @@ output_file_name: strings.g.dart
 translate_var: t
 enum_name: AppLocale
 class_name: Translations
+# Use {param} placeholders (translator-friendly, Weblate-compatible) instead of
+# slang's default Dart-style $param.
+string_interpolation: braces
 # Missing keys in a non-base locale fall back to the base locale at runtime.
 fallback_strategy: base_locale
 # Compile all locales in (not deferred), so setLocaleSync / useDeviceLocaleSync