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/lib/ui/variety_detail_screen.dart

@@ -1097,6 +1097,21 @@ class _SeedSavingView extends StatelessWidget {
           const SizedBox(height: 8),
           Text(note, style: const TextStyle(color: seedMuted, height: 1.35)),
         ],
+        // Advisory + a light source credit — the figures are general, not local
+        // gospel, and drawn from the seed-saving literature.
+        const SizedBox(height: 10),
+        Text(t.seedSaving.advisory,
+            style: const TextStyle(
+                color: seedMuted, fontSize: 12, fontStyle: FontStyle.italic)),
+        if (SeedSavingCatalog.sources case final sources when sources.isNotEmpty)
+          Padding(
+            padding: const EdgeInsets.only(top: 2),
+            child: Text(
+              '${t.seedSaving.sourcePrefix}: '
+              '${sources.map((s) => s.title).join(' · ')}',
+              style: const TextStyle(color: seedMuted, fontSize: 11),
+            ),
+          ),
       ],
     );
   }