feat(labels): print seed labels with QR from an inventory selection

Add multi-select to the inventory and a "Print labels" flow that renders a
PDF sheet of labels (small stickers or large cards), each carrying the seed's
common name, variety, scientific name, year/origin and a tanemaki://seed QR
that holds the seed's data offline (future scan-to-import).

- SeedLabelCodec: compact, versioned tanemaki://seed URI (encode/decode)
- LabelSheetService: pure-Dart PDF grid (pdf + barcode + DejaVu), RTL-aware
- VarietyRepository.labelRows: one row per lot, locale-picked common name
- InventoryCubit selection mode; contextual app bar + label print sheet
- i18n en/es/pt (printLabels.*)

Tests: codec, PDF service (both formats + RTL), labelRows, selection cubit,
inventory widget selection.

apps/app_seeds/test/ui/inventory_list_screen_test.dart

@@ -219,4 +219,66 @@ void main() {
       await disposeTree(tester);
     },
   );
+
+  testWidgets('the label toolbar enters selection and Select all enables print', (
+    tester,
+  ) async {
+    final repo = newTestRepository(db);
+    await repo.addQuickVariety(label: 'Tomato', category: 'Solanaceae');
+    await repo.addQuickVariety(label: 'Bean', category: 'Fabaceae');
+
+    await tester.pumpWidget(
+      wrapScreen(repository: repo, child: const InventoryListScreen()),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byKey(const Key('inventory.selectLabels')));
+    await tester.pumpAndSettle();
+
+    // Nothing picked yet: the count reads zero and print is disabled.
+    expect(find.text('0 selected'), findsOneWidget);
+    expect(
+      tester
+          .widget<IconButton>(find.byKey(const Key('inventory.selection.print')))
+          .onPressed,
+      isNull,
+    );
+
+    await tester.tap(find.byKey(const Key('inventory.selection.selectAll')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('2 selected'), findsOneWidget);
+    expect(
+      tester
+          .widget<IconButton>(find.byKey(const Key('inventory.selection.print')))
+          .onPressed,
+      isNotNull,
+    );
+
+    // Closing the contextual bar leaves selection mode.
+    await tester.tap(find.byKey(const Key('inventory.selection.close')));
+    await tester.pumpAndSettle();
+    expect(find.byKey(const Key('inventory.selection.print')), findsNothing);
+    await disposeTree(tester);
+  });
+
+  testWidgets('long-pressing a tile enters selection with it picked', (
+    tester,
+  ) async {
+    final repo = newTestRepository(db);
+    await repo.addQuickVariety(label: 'Tomato', category: 'Solanaceae');
+    await repo.addQuickVariety(label: 'Bean', category: 'Fabaceae');
+
+    await tester.pumpWidget(
+      wrapScreen(repository: repo, child: const InventoryListScreen()),
+    );
+    await tester.pumpAndSettle();
+
+    await tester.longPress(find.text('Tomato'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('1 selected'), findsOneWidget);
+    expect(find.byType(Checkbox), findsNWidgets(2)); // one per tile
+    await disposeTree(tester);
+  });
 }