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/ui/variety_detail_screen_test.dart

@@ -34,7 +34,7 @@ void main() {
 
     expect(find.text('Maize'), findsOneWidget); // app bar title
     expect(find.text('Poaceae'), findsOneWidget); // category chip
-    expect(find.text('Year 2024 · a cob'), findsOneWidget); // lot line
+    expect(find.text('Year 2024 · cob'), findsOneWidget); // lot line
     await disposeTree(tester);
   });
 
@@ -84,11 +84,13 @@ void main() {
     await tester.pumpAndSettle();
 
     await tester.enterText(find.byKey(const Key('addLot.year')), '2023');
-    await tester.tap(find.text('a handful'));
+    await tester.tap(find.text('handful')); // pick the unit from the scale
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byKey(const Key('quantity.count')), '2');
     await tester.tap(find.byKey(const Key('addLot.save')));
     await tester.pumpAndSettle();
 
-    expect(find.text('Year 2023 · a handful'), findsOneWidget);
+    expect(find.text('Year 2023 · 2 handfuls'), findsOneWidget);
     await disposeTree(tester);
   });