fix(i18n): translate remaining recipe card/detail strings, localize AI responses

Recipe cards, the recipe detail page, and meal planner still rendered raw
"{n} servings"/"{n}m cook"/"{n} srv" text and untranslated difficulty labels
outside the earlier i18n pass. Also wires the user's locale into AI features
that previously always answered in English regardless of app language:
recipe chat, ingredient substitution, recipe ideas, and generate-from-idea.
The recipe-language picker in the AI generate dialog now defaults to the
user's locale instead of always defaulting to English.

apps/web/messages/fr.json

@@ -323,6 +323,7 @@
     "addTo": "Ajouter à {day} – {meal}",
     "searchRecipes": "Rechercher des recettes…",
     "servings": "Portions",
+    "servingsAbbrev": "{count} pers.",
     "difficulty": "Difficulté",
     "noRecipes": "Aucune recette trouvée",
     "addFailed": "Échec de l'ajout",