feat(plantare): surface product link + dates; spec the bilateral record

The Plantaré screen stored madeOn/dueBy/varietyId but showed none of them,
so it felt half-built. Surface what's already there (no schema change):

- watchPlantareViews() joins each commitment with its variety label
- tile shows the made-on date, a "Return by {date}" line (amber + "overdue"
  when an open promise is past due, via new seedWarning colour), the variety
  name, and taps through to /variety/:id
- add sheet gains an optional return-by date picker (createPlantare already
  took dueBy)
- variety detail shows a read-only commitments section (hidden when empty,
  no add button — honours the single hand-over door)
- i18n en/es/pt/ast; tests for the join, dueBy round-trip, tile, and section

Also specs the deferred two-sided record in docs/design/plantare-bilateral.md
(Nostr pubkey counterparty, PlantareTransport propose->accept->counter-sign,
deferred key/signature/movement columns, provenance DAG, reputation anchor),
with a faithful transcription of the paper Plantaré v0.4 (BAH-Semillero 2009,
CC-BY-SA). Cross-linked from data-model §2.7 and sharing-model §6.

apps/app_seeds/test/ui/variety_detail_screen_test.dart

@@ -610,4 +610,35 @@ void main() {
     expect(find.byIcon(Icons.star_border), findsNothing);
     await disposeTree(tester);
   });
+
+  testWidgets('shows a commitments section only when the variety has one', (
+    tester,
+  ) async {
+    final repo = newTestRepository(db);
+    final id = await repo.addQuickVariety(label: 'Maize');
+
+    await tester.pumpWidget(
+      wrapDetail(
+        repository: repo,
+        varietyId: id,
+        species: newTestSpeciesRepository(db),
+      ),
+    );
+    await tester.pumpAndSettle();
+    // No commitments yet → the section stays hidden (progressive disclosure).
+    expect(find.byKey(const Key('detail.plantares')), findsNothing);
+
+    await repo.createPlantare(
+      direction: PlantareDirection.iReturn,
+      varietyId: id,
+      counterparty: 'Ana',
+      owedDescription: 'un puñado',
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.byKey(const Key('detail.plantares')), findsOneWidget);
+    expect(find.textContaining('un puñado'), findsOneWidget);
+    expect(find.textContaining('Ana'), findsOneWidget);
+    await disposeTree(tester);
+  });
 }