feat(inventory): informal quantity scale (number + unit) + seed/plant lots

Rework quantities the way seeds are actually described — a number of an informal
unit — and let a lot hold seeds or plants (plantel).

Quantity model (commons_core):
- Quantity is now { kind, count?, label } — "3 cobs", "2 packets", "a few".
  Count is optional; uncountable vibes (a few, some, pinch) carry no number.
- QuantityKind reorganised into an ascending scale: vibes → containers
  (teaspoon/spoon/cup/jar/sack) → seed & fruit forms → plant units, each tagged
  countable/not. Stored by name; count reuses the existing numeric column.

Lot type (schema v2):
- Lots gain a `type` (seed | plant); germination stays meaningful for seeds.
  schemaVersion 2 with a from1To2 migration (adds the column), drift_schema_v2
  exported, migration test covers v1→v2.

UI:
- New QuantityPicker: a horizontal scale of large seed-glyph icons + a number
  stepper (shown only for countable units), plus a seed/plant SegmentedButton.
  Used in quick-add and add-lot. Lot lines render "3 cobs" / "a few".
- i18n: unit singular/plural (ES/EN, Weblate-friendly), lot-type labels.

Build: slang moved to its CLI (disabled in build_runner to avoid a multi-file
collision); CI runs `dart run slang` before build_runner. 38 tests green,
Linux migrates the existing v1 DB and runs.

apps/app_seeds/lib/state/quick_add_cubit.dart

@@ -5,13 +5,17 @@ import 'package:equatable/equatable.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 
 import '../data/variety_repository.dart';
+import '../db/enums.dart';
+
+const Object _unset = Object();
 
 /// Form state for the quick-add sheet. Only [label] is required; everything
 /// else is progressive disclosure behind [expanded].
 class QuickAddState extends Equatable {
   const QuickAddState({
     this.label = '',
-    this.quantityKind,
+    this.quantity,
+    this.lotType = LotType.seed,
     this.photoBytes,
     this.expanded = false,
     this.submitting = false,
@@ -20,7 +24,8 @@ class QuickAddState extends Equatable {
   });
 
   final String label;
-  final QuantityKind? quantityKind;
+  final Quantity? quantity;
+  final LotType lotType;
   final Uint8List? photoBytes;
   final bool expanded;
   final bool submitting;
@@ -31,7 +36,8 @@ class QuickAddState extends Equatable {
 
   QuickAddState copyWith({
     String? label,
-    QuantityKind? quantityKind,
+    Object? quantity = _unset,
+    LotType? lotType,
     Uint8List? photoBytes,
     bool? expanded,
     bool? submitting,
@@ -40,7 +46,10 @@ class QuickAddState extends Equatable {
   }) {
     return QuickAddState(
       label: label ?? this.label,
-      quantityKind: quantityKind ?? this.quantityKind,
+      quantity: identical(quantity, _unset)
+          ? this.quantity
+          : quantity as Quantity?,
+      lotType: lotType ?? this.lotType,
       photoBytes: photoBytes ?? this.photoBytes,
       expanded: expanded ?? this.expanded,
       submitting: submitting ?? this.submitting,
@@ -52,7 +61,8 @@ class QuickAddState extends Equatable {
   @override
   List<Object?> get props => [
     label,
-    quantityKind,
+    quantity,
+    lotType,
     photoBytes,
     expanded,
     submitting,
@@ -70,8 +80,10 @@ class QuickAddCubit extends Cubit<QuickAddState> {
   void labelChanged(String value) =>
       emit(state.copyWith(label: value, showLabelError: false));
 
-  void selectQuantity(QuantityKind kind) =>
-      emit(state.copyWith(quantityKind: kind));
+  void setQuantity(Quantity? quantity) =>
+      emit(state.copyWith(quantity: quantity));
+
+  void setLotType(LotType type) => emit(state.copyWith(lotType: type));
 
   void photoPicked(Uint8List bytes) => emit(state.copyWith(photoBytes: bytes));
 
@@ -87,9 +99,8 @@ class QuickAddCubit extends Cubit<QuickAddState> {
     emit(state.copyWith(submitting: true));
     await _repo.addQuickVariety(
       label: state.label.trim(),
-      quantity: state.quantityKind == null
-          ? null
-          : Quantity(kind: state.quantityKind!),
+      lotType: state.lotType,
+      quantity: state.quantity,
       photoBytes: state.photoBytes,
     );
     emit(state.copyWith(submitting: false, submitted: true));