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/data/export_import/seed_label_codec_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/data/export_import/seed_label_codec.dart';
+
+void main() {
+  group('SeedLabelCodec.encode', () {
+    test('encodes all fields as a tanemaki://seed URI', () {
+      final uri = SeedLabelCodec.encode(
+        const SeedLabelData(
+          varietyLabel: 'Acelga de Perales',
+          scientificName: 'Beta vulgaris',
+          commonName: 'Acelga',
+          year: 2006,
+          origin: 'Perales',
+        ),
+      );
+      expect(uri, startsWith('tanemaki://seed?'));
+      final parsed = Uri.parse(uri);
+      expect(parsed.scheme, 'tanemaki');
+      expect(parsed.host, 'seed');
+      expect(parsed.queryParameters, {
+        'v': 'Acelga de Perales',
+        's': 'Beta vulgaris',
+        'c': 'Acelga',
+        'y': '2006',
+        'o': 'Perales',
+      });
+    });
+
+    test('omits absent fields, keeping the payload small', () {
+      final uri = SeedLabelCodec.encode(
+        const SeedLabelData(varietyLabel: 'Alubia de Tolosa'),
+      );
+      final parsed = Uri.parse(uri);
+      expect(parsed.queryParameters, {'v': 'Alubia de Tolosa'});
+      expect(uri, isNot(contains('&s=')));
+      expect(uri, isNot(contains('y=')));
+    });
+
+    test('treats blank optional fields as absent', () {
+      final uri = SeedLabelCodec.encode(
+        const SeedLabelData(
+          varietyLabel: 'Maíz',
+          scientificName: '   ',
+          origin: '',
+        ),
+      );
+      expect(Uri.parse(uri).queryParameters, {'v': 'Maíz'});
+    });
+  });
+
+  group('SeedLabelCodec round-trip', () {
+    test('decode reverses encode for full data', () {
+      const data = SeedLabelData(
+        varietyLabel: 'Tomate Rosa',
+        scientificName: 'Solanum lycopersicum',
+        commonName: 'Tomate',
+        year: 2024,
+        origin: 'Huerta de la abuela, León',
+      );
+      expect(SeedLabelCodec.decode(SeedLabelCodec.encode(data)), data);
+    });
+
+    test('survives unicode, spaces and separators', () {
+      const data = SeedLabelData(
+        varietyLabel: 'Grelo & nabo "do país"',
+        origin: 'A Coruña / Galiza',
+        commonName: 'بامية', // RTL script
+      );
+      final decoded = SeedLabelCodec.decode(SeedLabelCodec.encode(data));
+      expect(decoded, data);
+    });
+  });
+
+  group('SeedLabelCodec.decode', () {
+    test('rejects a foreign scheme', () {
+      expect(SeedLabelCodec.decode('https://seed?v=x'), isNull);
+      expect(SeedLabelCodec.decode('tanemaki://other?v=x'), isNull);
+    });
+
+    test('rejects a payload without a variety', () {
+      expect(SeedLabelCodec.decode('tanemaki://seed?s=Beta%20vulgaris'), isNull);
+    });
+
+    test('ignores an unparseable year rather than throwing', () {
+      final decoded = SeedLabelCodec.decode('tanemaki://seed?v=Maíz&y=old');
+      expect(decoded?.varietyLabel, 'Maíz');
+      expect(decoded?.year, isNull);
+    });
+  });
+}