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/en.i18n.json

@@ -45,6 +45,10 @@
     "noLots": "No lots yet.",
     "names": "Also known as",
     "addName": "Add name",
+    "links": "Links",
+    "addLink": "Add link",
+    "linkUrl": "URL",
+    "linkTitle": "Title (optional)",
     "notes": "Notes",
     "addLot": "Add lot",
     "editLot": "Edit lot",