feat: @mentions in comments

Parse @username in comment content, notify mentioned users (dedup
against recipe-author/parent-author who are already notified via
comment/reply), and render @username as a link to their profile.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/messages/en.json

@@ -25,7 +25,8 @@
     "comment": "{name} commented on your recipe",
     "reply": "{name} replied to your comment",
     "reaction": "{name} reacted to your comment",
-    "rating": "{name} rated your recipe"
+    "rating": "{name} rated your recipe",
+    "mention": "{name} mentioned you in a comment"
   },
   "recipe": {
     "share": "Share",

apps/web/messages/fr.json

@@ -25,7 +25,8 @@
     "comment": "{name} a commenté votre recette",
     "reply": "{name} a répondu à votre commentaire",
     "reaction": "{name} a réagi à votre commentaire",
-    "rating": "{name} a noté votre recette"
+    "rating": "{name} a noté votre recette",
+    "mention": "{name} vous a mentionné dans un commentaire"
   },
   "recipe": {
     "share": "Partager",