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/domain/seed_saving.dart

@@ -86,12 +86,21 @@ class SeedSavingGuide {
   }
 }
 
+/// An attribution for the bundled guidance (the facts are drawn from these).
+class SeedSavingSource {
+  const SeedSavingSource({required this.title, this.author, this.url});
+  final String title;
+  final String? author;
+  final String? url;
+}
+
 /// The whole bundled dataset: family defaults plus per-taxon overrides (keyed by
 /// scientific name OR bare genus). Lookups are case-insensitive.
 class SeedSavingData {
   SeedSavingData({
     required Map<String, SeedSavingGuide> families,
     required Map<String, SeedSavingGuide> taxa,
+    this.sources = const [],
   })  : _families = {
           for (final e in families.entries) e.key.toLowerCase().trim(): e.value,
         },
@@ -102,6 +111,9 @@ class SeedSavingData {
   final Map<String, SeedSavingGuide> _families;
   final Map<String, SeedSavingGuide> _taxa;
 
+  /// Where the guidance comes from — shown as a small credit under the section.
+  final List<SeedSavingSource> sources;
+
   /// The best guide for a crop: the family default, overlaid by a per-species
   /// (or per-genus) entry when one exists. Returns null when nothing matches.
   SeedSavingGuide? guideFor({String? scientificName, String? family}) {
@@ -151,5 +163,18 @@ SeedSavingData parseSeedSaving(String jsonString) {
     );
   }
 
-  return SeedSavingData(families: section('families'), taxa: section('species'));
+  final sources = [
+    for (final s in (root['sources'] as List? ?? const []).cast<Map>())
+      SeedSavingSource(
+        title: s['title'] as String,
+        author: s['author'] as String?,
+        url: s['url'] as String?,
+      ),
+  ];
+
+  return SeedSavingData(
+    families: section('families'),
+    taxa: section('species'),
+    sources: sources,
+  );
 }