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/lib/ui/plantare_sheet.dart

@@ -34,6 +34,7 @@ class _PlantareSheetState extends State<_PlantareSheet> {
   final _counterparty = TextEditingController();
   final _owed = TextEditingController();
   final _note = TextEditingController();
+  DateTime? _dueBy;
   bool _saving = false;
 
   @override
@@ -46,6 +47,17 @@ class _PlantareSheetState extends State<_PlantareSheet> {
 
   String? _nullIfBlank(String s) => s.trim().isEmpty ? null : s.trim();
 
+  Future<void> _pickDueBy() async {
+    final now = DateTime.now();
+    final picked = await showDatePicker(
+      context: context,
+      initialDate: _dueBy ?? DateTime(now.year + 1, now.month, now.day),
+      firstDate: DateTime(now.year - 1),
+      lastDate: DateTime(now.year + 10),
+    );
+    if (picked != null) setState(() => _dueBy = picked);
+  }
+
   Future<void> _save() async {
     setState(() => _saving = true);
     await widget.repository.createPlantare(
@@ -53,6 +65,7 @@ class _PlantareSheetState extends State<_PlantareSheet> {
       varietyId: widget.varietyId,
       counterparty: _nullIfBlank(_counterparty.text),
       owedDescription: _nullIfBlank(_owed.text),
+      dueBy: _dueBy?.millisecondsSinceEpoch,
       note: _nullIfBlank(_note.text),
     );
     if (mounted) Navigator.of(context).pop();
@@ -121,6 +134,43 @@ class _PlantareSheetState extends State<_PlantareSheet> {
               ),
             ),
             const SizedBox(height: 12),
+            // Optional return-by date — a gentle reminder, never enforced.
+            InputDecorator(
+              decoration: InputDecoration(
+                labelText: t.plantare.dueByLabel,
+                helperText: t.plantare.dueByHint,
+                border: const OutlineInputBorder(),
+              ),
+              child: Row(
+                children: [
+                  Expanded(
+                    child: Text(
+                      _dueBy == null
+                          ? t.plantare.pickDate
+                          : MaterialLocalizations.of(context)
+                              .formatShortDate(_dueBy!),
+                      style: TextStyle(
+                        color: _dueBy == null ? seedMuted : seedOnSurface,
+                      ),
+                    ),
+                  ),
+                  if (_dueBy != null)
+                    IconButton(
+                      key: const Key('plantare.dueBy.clear'),
+                      icon: const Icon(Icons.clear, size: 20),
+                      tooltip: t.plantare.clearDate,
+                      onPressed: () => setState(() => _dueBy = null),
+                    ),
+                  IconButton(
+                    key: const Key('plantare.dueBy.pick'),
+                    icon: const Icon(Icons.event_outlined),
+                    tooltip: t.plantare.pickDate,
+                    onPressed: _pickDueBy,
+                  ),
+                ],
+              ),
+            ),
+            const SizedBox(height: 12),
             TextField(
               key: const Key('plantare.note'),
               controller: _note,