feat(i18n): locale-aware formatters and useLanguage hook

- Create useLanguage() hook following useCurrency/useWeightUnit pattern
- Update formatPrice() to use Intl.NumberFormat for locale-aware currency display
- Update formatWeight() to use Intl.NumberFormat for locale-aware number formatting
- Update formatDualPrice() to pass locale through
- Update useFormatters() to pass locale to all formatters
- Add formatter tests for en/de locales (15 tests passing)

Phase 34, Plan 03

src/client/hooks/useFormatters.ts

@@ -1,14 +1,17 @@
 import { formatPrice, formatWeight } from "../lib/formatters";
 import { useCurrency } from "./useCurrency";
+import { useLanguage } from "./useLanguage";
 import { useWeightUnit } from "./useWeightUnit";
 
 export function useFormatters() {
 	const unit = useWeightUnit();
 	const { currency } = useCurrency();
+	const locale = useLanguage();
 	return {
-		weight: (grams: number | null) => formatWeight(grams, unit),
-		price: (cents: number | null) => formatPrice(cents, currency),
+		weight: (grams: number | null) => formatWeight(grams, unit, locale),
+		price: (cents: number | null) => formatPrice(cents, currency, locale),
 		unit,
 		currency,
+		locale,
 	};
 }

src/client/hooks/useLanguage.ts

@@ -0,0 +1,12 @@
+import { useSetting } from "./useSettings";
+
+export const VALID_LANGUAGES = ["en", "de"] as const;
+export type Language = (typeof VALID_LANGUAGES)[number];
+
+export function useLanguage(): Language {
+	const { data } = useSetting("language");
+	if (data && VALID_LANGUAGES.includes(data as Language)) {
+		return data as Language;
+	}
+	return "en";
+}