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/es.i18n.json

@@ -45,6 +45,10 @@
     "noLots": "Aún no hay lotes.",
     "names": "También conocida como",
     "addName": "Añadir nombre",
+    "links": "Enlaces",
+    "addLink": "Añadir enlace",
+    "linkUrl": "URL",
+    "linkTitle": "Título (opcional)",
     "notes": "Notas",
     "addLot": "Añadir lote",
     "editLot": "Editar lote",