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/test/data/variety_detail_test.dart

@@ -212,4 +212,20 @@ void main() {
 
     await queue.cancel();
   });
+
+  test('add and remove external links', () async {
+    final id = await repo.addQuickVariety(label: 'Maize');
+    final linkId = await repo.addExternalLink(
+      id,
+      'https://en.wikipedia.org/wiki/Maize',
+      title: 'Wikipedia',
+    );
+
+    final detail = await repo.watchVariety(id).first;
+    expect(detail!.links.single.url, 'https://en.wikipedia.org/wiki/Maize');
+    expect(detail.links.single.display, 'Wikipedia'); // title preferred
+
+    await repo.removeExternalLink(linkId);
+    expect((await repo.watchVariety(id).first)!.links, isEmpty);
+  });
 }