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/theme_contrast_test.dart

@@ -48,5 +48,10 @@ void main() {
       expectAa(Colors.white, seedGreen, 'white on primary green');
       expectAa(Colors.white, seedAppBar, 'white on app-bar green');
     });
+
+    test('overdue warning text on every background it sits on', () {
+      expectAa(seedWarning, Colors.white, 'seedWarning on white');
+      expectAa(seedWarning, seedCanvas, 'seedWarning on canvas');
+    });
   });
 }