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/lib/data/export_import/seed_label_codec.dart

@@ -0,0 +1,92 @@
+import 'package:equatable/equatable.dart';
+
+/// The seed facts a printed label carries in its QR code — just enough to
+/// identify the seed offline, so the data travels on the physical packet with
+/// no server and no account. Kept deliberately small so the QR stays scannable
+/// at sticker size.
+class SeedLabelData extends Equatable {
+  const SeedLabelData({
+    required this.varietyLabel,
+    this.scientificName,
+    this.commonName,
+    this.year,
+    this.origin,
+  });
+
+  /// The variety's label, as its keeper writes it (e.g. "Acelga de Perales").
+  final String varietyLabel;
+
+  /// Scientific name of the linked species, if any (e.g. "Beta vulgaris").
+  final String? scientificName;
+
+  /// The species' common name in the keeper's locale (e.g. "Acelga").
+  final String? commonName;
+
+  /// Harvest year of the batch, if known.
+  final int? year;
+
+  /// Where the seed came from (origin name and/or place), if recorded.
+  final String? origin;
+
+  @override
+  List<Object?> get props => [
+    varietyLabel,
+    scientificName,
+    commonName,
+    year,
+    origin,
+  ];
+}
+
+/// Encodes/decodes [SeedLabelData] as a compact, versioned `tanemaki://seed`
+/// URI — self-describing and parseable, so a future scan-to-import feature can
+/// read a label a keeper printed today. Unicode and RTL text are percent-encoded
+/// by [Uri], so any QR reader round-trips them safely.
+///
+/// Shape: `tanemaki://seed?v=<variety>&s=<scientific>&c=<common>&y=<year>&o=<origin>`
+/// Absent fields are omitted, keeping the payload (and the QR) as small as the
+/// data allows.
+abstract final class SeedLabelCodec {
+  static const _scheme = 'tanemaki';
+  static const _host = 'seed';
+
+  static String encode(SeedLabelData data) {
+    final params = <String, String>{
+      'v': data.varietyLabel,
+      if (_present(data.scientificName)) 's': data.scientificName!.trim(),
+      if (_present(data.commonName)) 'c': data.commonName!.trim(),
+      if (data.year != null) 'y': '${data.year}',
+      if (_present(data.origin)) 'o': data.origin!.trim(),
+    };
+    return Uri(
+      scheme: _scheme,
+      host: _host,
+      queryParameters: params,
+    ).toString();
+  }
+
+  /// Parses a `tanemaki://seed` URI back to [SeedLabelData]. Returns null when
+  /// [input] is not a well-formed seed label (wrong scheme/host, or no variety).
+  static SeedLabelData? decode(String input) {
+    final uri = Uri.tryParse(input.trim());
+    if (uri == null || uri.scheme != _scheme || uri.host != _host) return null;
+    final q = uri.queryParameters;
+    final variety = q['v']?.trim();
+    if (variety == null || variety.isEmpty) return null;
+    final year = q['y'];
+    return SeedLabelData(
+      varietyLabel: variety,
+      scientificName: _clean(q['s']),
+      commonName: _clean(q['c']),
+      year: year == null ? null : int.tryParse(year),
+      origin: _clean(q['o']),
+    );
+  }
+
+  static bool _present(String? value) => value != null && value.trim().isNotEmpty;
+
+  static String? _clean(String? value) {
+    final trimmed = value?.trim();
+    return (trimmed == null || trimmed.isEmpty) ? null : trimmed;
+  }
+}