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/i18n/strings_en.g.dart

@@ -163,6 +163,12 @@ class Translations$seedSaving$en {
 
 	/// en: 'Hard'
 	String get diffHard => 'Hard';
+
+	/// en: 'General guidance — adapt it to your climate and variety.'
+	String get advisory => 'General guidance — adapt it to your climate and variety.';
+
+	/// en: 'Source'
+	String get sourcePrefix => 'Source';
 }
 
 // Path: calendar
@@ -2251,6 +2257,8 @@ extension on Translations {
 			'seedSaving.diffEasy' => 'Easy',
 			'seedSaving.diffMedium' => 'Medium',
 			'seedSaving.diffHard' => 'Hard',
+			'seedSaving.advisory' => 'General guidance — adapt it to your climate and variety.',
+			'seedSaving.sourcePrefix' => 'Source',
 			'calendar.title' => 'This month',
 			'calendar.filterChip' => 'This month',
 			'calendar.nothing' => ({required Object month}) => 'Nothing noted for ${month}.',