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/test/db/schema/schema.dart

@@ -5,6 +5,7 @@
 import 'package:drift/drift.dart';
 import 'package:drift/internal/migrations.dart';
 import 'schema_v1.dart' as v1;
+import 'schema_v2.dart' as v2;
 
 class GeneratedHelper implements SchemaInstantiationHelper {
   @override
@@ -12,10 +13,12 @@ class GeneratedHelper implements SchemaInstantiationHelper {
     switch (version) {
       case 1:
         return v1.DatabaseAtV1(db);
+      case 2:
+        return v2.DatabaseAtV2(db);
       default:
         throw MissingSchemaException(version, versions);
     }
   }
 
-  static const versions = const [1];
+  static const versions = const [1, 2];
 }