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/data/plantare_test.dart

@@ -61,6 +61,34 @@ void main() {
     expect(await repo.watchPlantares().first, isEmpty);
   });
 
+  test('watchPlantareViews joins the linked variety label (null when unlinked)',
+      () async {
+    final repo = newTestRepository(db);
+    final vid = await repo.addQuickVariety(label: 'Tomate rosa');
+    await repo.createPlantare(
+      direction: PlantareDirection.iReturn,
+      varietyId: vid,
+    );
+    await repo.createPlantare(direction: PlantareDirection.owedToMe);
+
+    final views = await repo.watchPlantareViews().first;
+    expect(views, hasLength(2));
+    final linked = views.firstWhere((v) => v.plantare.varietyId == vid);
+    expect(linked.varietyLabel, 'Tomate rosa');
+    final unlinked = views.firstWhere((v) => v.plantare.varietyId == null);
+    expect(unlinked.varietyLabel, isNull);
+  });
+
+  test('a return-by date round-trips through create', () async {
+    final repo = newTestRepository(db);
+    final due = DateTime(2027, 3, 1).millisecondsSinceEpoch;
+    await repo.createPlantare(
+      direction: PlantareDirection.iReturn,
+      dueBy: due,
+    );
+    expect((await repo.watchPlantares().first).single.dueBy, due);
+  });
+
   test('commitments survive a backup round-trip (in exportInventory/import)',
       () async {
     final repoA = newTestRepository(db);