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/substitute/route.ts

@@ -41,10 +41,13 @@ export async function POST(req: NextRequest) {
     ? { provider: parsed.data.provider, model: parsed.data.model }
     : await getDefaultProviderWithKey(session!.user.id);
 
+  const locale = (session!.user as { locale?: string }).locale ?? "en";
+
   const substitutions = await substituteIngredient(
     parsed.data.ingredient,
     context,
-    aiConfig
+    aiConfig,
+    locale
   );
 
   return NextResponse.json({ substitutions });