refactor: improve locale modularity

- Added src/lib/i18n/metadata.ts with helper functions
- Updated [locale]/layout.tsx to use DEFAULT_LOCALE constant
- routing.ts already uses centralized SUPPORTED_LOCALES

Note: For full antifragility, a larger refactor would centralize
all hardcoded locale comparisons for metadata text fallbacks.
Currently adding a new language requires:
1. SUPPORTED_LOCALES in locales.ts
2. LOCALE_CONFIG entry
3. Translation keys in all message files

src/lib/i18n/metadata.ts

@@ -0,0 +1,17 @@
+import { DEFAULT_LOCALE, LOCALE_CONFIG, type Locale } from "./locales";
+
+export function getMetadataText(locale: Locale, texts: Partial<Record<Locale, string>>): string {
+  return texts[locale] || texts[DEFAULT_LOCALE] || "";
+}
+
+export function getSaleorLocale(locale: Locale): string {
+  return LOCALE_CONFIG[locale].saleorLocale;
+}
+
+export function getLocaleDisplayName(locale: Locale): string {
+  return LOCALE_CONFIG[locale].label;
+}
+
+export function isDefaultLocale(locale: string): boolean {
+  return locale === DEFAULT_LOCALE;
+}