feat(inventory): external links on the variety detail

Add Wikipedia/forum-style URLs to a variety:
- Repository: addExternalLink/removeExternalLink (soft delete); watchVariety
  loads links and re-emits on external_links changes; ExternalLinkItem model
  (title preferred over URL for display).
- Cubit: addExternalLink/removeExternalLink.
- Detail: a "Links" section with tappable link chips (open via url_launcher,
  http(s):// prepended if missing), delete per chip, and an "Add link" dialog
  (URL + optional title).

Tests: repo add/remove + widget add/remove. 52 green; Linux runs.

apps/app_seeds/lib/i18n/strings_en.g.dart

@@ -216,6 +216,18 @@ class Translations$detail$en {
 	/// en: 'Add name'
 	String get addName => 'Add name';
 
+	/// en: 'Links'
+	String get links => 'Links';
+
+	/// en: 'Add link'
+	String get addLink => 'Add link';
+
+	/// en: 'URL'
+	String get linkUrl => 'URL';
+
+	/// en: 'Title (optional)'
+	String get linkTitle => 'Title (optional)';
+
 	/// en: 'Notes'
 	String get notes => 'Notes';
 
@@ -757,6 +769,10 @@ extension on Translations {
 			'detail.noLots' => 'No lots yet.',
 			'detail.names' => 'Also known as',
 			'detail.addName' => 'Add name',
+			'detail.links' => 'Links',
+			'detail.addLink' => 'Add link',
+			'detail.linkUrl' => 'URL',
+			'detail.linkTitle' => 'Title (optional)',
 			'detail.notes' => 'Notes',
 			'detail.addLot' => 'Add lot',
 			'detail.editLot' => 'Edit lot',