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.

packages/commons_core/test/value/quantity_test.dart

@@ -4,25 +4,22 @@ import 'package:test/test.dart';
 void main() {
   group('Quantity', () {
     test('value equality ignores object identity', () {
-      const a = Quantity(kind: QuantityKind.pod, precise: 12, label: 'a bag');
-      const b = Quantity(kind: QuantityKind.pod, precise: 12, label: 'a bag');
+      const a = Quantity(kind: QuantityKind.pod, count: 3, label: 'a bag');
+      const b = Quantity(kind: QuantityKind.pod, count: 3, label: 'a bag');
       expect(a, b);
       expect(a.hashCode, b.hashCode);
     });
 
-    test('isPrecise reflects the presence of a precise amount', () {
-      expect(
-        const Quantity(kind: QuantityKind.grams, precise: 5).isPrecise,
-        isTrue,
-      );
-      expect(const Quantity(kind: QuantityKind.aFew).isPrecise, isFalse);
+    test('hasCount reflects the presence of a number', () {
+      expect(const Quantity(kind: QuantityKind.cob, count: 3).hasCount, isTrue);
+      expect(const Quantity(kind: QuantityKind.aFew).hasCount, isFalse);
     });
 
     test('copyWith overrides only the given fields', () {
-      const q = Quantity(kind: QuantityKind.cob, precise: 2);
+      const q = Quantity(kind: QuantityKind.cob, count: 2);
       final q2 = q.copyWith(kind: QuantityKind.ear);
       expect(q2.kind, QuantityKind.ear);
-      expect(q2.precise, 2);
+      expect(q2.count, 2);
     });
   });
 
@@ -31,11 +28,20 @@ void main() {
       // Guards against accidental renames — these strings live in the DB.
       expect(QuantityKind.pod.name, 'pod');
       expect(QuantityKind.cob.name, 'cob');
-      expect(QuantityKind.head.name, 'head');
       expect(QuantityKind.packet.name, 'packet');
       expect(QuantityKind.handful.name, 'handful');
-      expect(QuantityKind.grams.name, 'grams');
-      expect(QuantityKind.count.name, 'count');
+      expect(QuantityKind.jar.name, 'jar');
+      expect(QuantityKind.sack.name, 'sack');
+      expect(QuantityKind.plant.name, 'plant');
+    });
+
+    test('vibe amounts are not countable; forms and containers are', () {
+      expect(QuantityKind.aFew.countable, isFalse);
+      expect(QuantityKind.some.countable, isFalse);
+      expect(QuantityKind.cob.countable, isTrue);
+      expect(QuantityKind.packet.countable, isTrue);
+      expect(QuantityKind.jar.countable, isTrue);
+      expect(QuantityKind.plant.countable, isTrue);
     });
 
     test('every kind belongs to a group', () {