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/state/inventory_cubit.dart

@@ -20,6 +20,8 @@ class InventoryState extends Equatable {
     this.needsReproductionOnly = false,
     this.sharingOnly = false,
     this.loading = true,
+    this.selectionMode = false,
+    this.selectedIds = const {},
   });
 
   final List<VarietyListItem> items;
@@ -49,6 +51,13 @@ class InventoryState extends Equatable {
 
   final bool loading;
 
+  /// Whether the list is in multi-select mode — used to pick a subset of the
+  /// inventory to print labels for.
+  final bool selectionMode;
+
+  /// Ids of the varieties currently selected in [selectionMode].
+  final Set<String> selectedIds;
+
   /// Categories present across all items, in display order (deduped), so the
   /// UI can offer one chip per category actually in use.
   List<String> get categories {
@@ -101,6 +110,8 @@ class InventoryState extends Equatable {
     bool? needsReproductionOnly,
     bool? sharingOnly,
     bool? loading,
+    bool? selectionMode,
+    Set<String>? selectedIds,
   }) {
     return InventoryState(
       items: items ?? this.items,
@@ -113,6 +124,8 @@ class InventoryState extends Equatable {
           needsReproductionOnly ?? this.needsReproductionOnly,
       sharingOnly: sharingOnly ?? this.sharingOnly,
       loading: loading ?? this.loading,
+      selectionMode: selectionMode ?? this.selectionMode,
+      selectedIds: selectedIds ?? this.selectedIds,
     );
   }
 
@@ -127,6 +140,8 @@ class InventoryState extends Equatable {
     needsReproductionOnly,
     sharingOnly,
     loading,
+    selectionMode,
+    selectedIds,
   ];
 }
 
@@ -177,6 +192,36 @@ class InventoryCubit extends Cubit<InventoryState> {
   void toggleSharingOnly() =>
       emit(state.copyWith(sharingOnly: !state.sharingOnly));
 
+  /// Enters multi-select mode with an empty selection (from the toolbar).
+  void startSelection() =>
+      emit(state.copyWith(selectionMode: true, selectedIds: const {}));
+
+  /// Enters multi-select mode with [id] as the first selected variety
+  /// (triggered by a long-press on a tile).
+  void enterSelection(String id) =>
+      emit(state.copyWith(selectionMode: true, selectedIds: {id}));
+
+  /// Toggles a variety in the selection (add if absent, remove if present).
+  /// An empty selection stays in selection mode; the user exits explicitly.
+  void toggleSelection(String id) {
+    final next = Set<String>.of(state.selectedIds);
+    if (!next.remove(id)) next.add(id);
+    emit(state.copyWith(selectionMode: true, selectedIds: next));
+  }
+
+  /// Selects every currently visible variety, so "filter to a category, then
+  /// select all" is the way to pick a subset of the inventory to print.
+  void selectAllVisible() => emit(
+    state.copyWith(
+      selectionMode: true,
+      selectedIds: state.visibleItems.map((i) => i.id).toSet(),
+    ),
+  );
+
+  /// Leaves selection mode and clears the selection.
+  void exitSelection() =>
+      emit(state.copyWith(selectionMode: false, selectedIds: const {}));
+
   /// Clears all filters (search is left untouched).
   void clearFilters() => emit(
     state.copyWith(