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/lib/ai/features/substitute-ingredient.ts

@@ -13,18 +13,22 @@ const SubstitutionsSchema = z.object({
 
 export type Substitution = z.infer<typeof SubstitutionsSchema>["substitutions"][number];
 
+const LANG: Record<string, string> = { en: "English", fr: "French" };
+
 export async function substituteIngredient(
   ingredient: string,
   recipeContext: string,
-  config?: AiConfig
+  config?: AiConfig,
+  locale?: string
 ): Promise<Substitution[]> {
   const model = resolveModel(config);
+  const lang = LANG[locale ?? "en"] ?? "English";
 
   const { object } = await generateObject({
     model,
     schema: SubstitutionsSchema,
     system:
-      "You are a professional chef specializing in ingredient substitutions. Provide practical, commonly available substitutes. For each substitute: name what to use, the ratio (e.g. '1:1', '3/4 the amount'), and a brief note on technique adjustments. Rate the flavor impact honestly.",
+      `You are a professional chef specializing in ingredient substitutions. Provide practical, commonly available substitutes. For each substitute: name what to use, the ratio (e.g. '1:1', '3/4 the amount'), and a brief note on technique adjustments. Rate the flavor impact honestly. Respond in ${lang}.`,
     prompt: `Suggest 3-4 substitutes for "${ingredient}" in this recipe context: ${recipeContext}`,
   });