feat(block1): photo thumbnails in the inventory list

The list showed only initials even when a variety had a photo (quick-add stores
one as an encrypted BLOB). Now the avatar shows the first photo, falling back to
the initial — matching the "foto/inicial" list spec.

- VarietyListItem carries the first photo bytes; watchInventory loads them in a
  single query per emission.
- List tile renders a MemoryImage avatar when present.

Tests: repository (photo bytes surfaced, null when none) and a widget test
(avatar replaces the initial). Full suite: 37 passing, 0 skipped.

apps/app_seeds/test/data/variety_repository_test.dart

@@ -52,6 +52,18 @@ void main() {
     },
   );
 
+  test('watchInventory carries the first photo bytes for the avatar', () async {
+    await repo.addQuickVariety(
+      label: 'With photo',
+      photoBytes: Uint8List.fromList([9, 8, 7]),
+    );
+    await repo.addQuickVariety(label: 'No photo');
+
+    final items = await repo.watchInventory().first;
+    expect(items.firstWhere((i) => i.label == 'With photo').photo, [9, 8, 7]);
+    expect(items.firstWhere((i) => i.label == 'No photo').photo, isNull);
+  });
+
   test(
     'stream is ordered by category then label and excludes soft-deleted rows',
     () async {