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/app/api/v1/ai/recipe-chat/route.ts

@@ -14,6 +14,8 @@ const Schema = z.object({
   question: z.string().min(1).max(500),
 });
 
+const LANG: Record<string, string> = { en: "English", fr: "French" };
+
 export async function POST(req: NextRequest) {
   const { session, response } = await requireSession();
   if (response) return response;
@@ -66,10 +68,12 @@ ${stepList || "None listed"}
   ]);
   const model = resolveModel(config);
   const bioContext = buildUserBioContext(privateBio);
+  const locale = (session!.user as { locale?: string }).locale ?? "en";
+  const lang = LANG[locale] ?? "English";
 
   const { text } = await generateText({
     model,
-    system: `You are a helpful culinary assistant. Answer questions about the following recipe concisely and accurately. If a question is not related to the recipe or cooking, politely redirect. Keep answers under 200 words.
+    system: `You are a helpful culinary assistant. Answer questions about the following recipe concisely and accurately. If a question is not related to the recipe or cooking, politely redirect. Keep answers under 200 words. Respond in ${lang}.
 
 ${recipeContext}${bioContext}`,
     prompt: parsed.data.question,