feat(seed-saving): source attribution + advisory note

Record where the guidance comes from (copyright/attribution) and make
clear it is general, not local gospel — kept light:
- seed_saving.json gains a top-level sources list (Seed to Seed / Seed
  Savers Exchange / Organic Seed Alliance); parsed into
  SeedSavingData.sources, exposed via SeedSavingCatalog.sources.
- The detail section shows a small muted footer: an italic 'advisory —
  adapt to your climate' line plus a 'Source: …' credit.
- i18n seedSaving.advisory + sourcePrefix (en/es/pt/ast).
- Tests: asset carries sources; the section renders advisory + credit.

apps/app_seeds/test/data/seed_saving_asset_test.dart

@@ -48,4 +48,9 @@ void main() {
     expect(g?.vector, PollinationVector.wind);
     expect(g?.recommendedPlants, greaterThanOrEqualTo(50));
   });
+
+  test('carries source attributions', () {
+    expect(data.sources, isNotEmpty);
+    expect(data.sources.map((s) => s.title), contains('Seed to Seed'));
+  });
 }