feat(plantare): tie the propose form to a real seed

The propose sheet used a free-text seed name, unlinked from the
inventory. Now the seed is picked from your catalogued varieties so the
Plantaré is always tied to a real Variety (tap-through to its detail, and
it shows in that seed's commitments):

- Seed field is an Autocomplete over the inventory; on send the typed name
  resolves to the matching Variety, or creates a quick one if it's new —
  so varietyId is never null. Sheet takes the VarietyRepository; chat
  passes it.
- New repo one-shot `varietyLabels()` (id + label, no joins) feeds the
  picker. Deliberately a Future, not `watchInventory()` — a transient
  sheet must not hold a live Drift subscription (that hung the widget test
  ~36 min). Tests use it too and now run in ~2s with bounded pumps + a
  tall surface, no pumpAndSettle.

Tests: a new name creates & links a Variety; a matching name links the
existing one without duplicating. ui/services/data plantare suites green.

apps/app_seeds/lib/data/variety_repository.dart

@@ -619,6 +619,20 @@ class VarietyRepository {
     );
   }
 
+  /// One-shot list of catalogued (named, non-draft) seeds for pickers — id +
+  /// label only, no photo/species joins. A Future (not a stream) so a transient
+  /// sheet doesn't hold a live subscription (which would hang widget tests).
+  Future<List<({String id, String label})>> varietyLabels() async {
+    final rows = await (_db.select(_db.varieties)
+          ..where((v) => v.isDeleted.equals(false) & v.isDraft.equals(false))
+          ..orderBy([(v) => OrderingTerm(expression: v.label)]))
+        .get();
+    return [
+      for (final v in rows)
+        if (v.label.trim().isNotEmpty) (id: v.id, label: v.label),
+    ];
+  }
+
   Future<List<VarietyListItem>> _loadInventory() async {
     final rows =
         await (_db.select(_db.varieties)