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/app/[locale]/layout.tsx

@@ -1,7 +1,7 @@
 import { Metadata } from "next";
 import { NextIntlClientProvider } from "next-intl";
 import { getMessages, setRequestLocale } from "next-intl/server";
-import { SUPPORTED_LOCALES } from "@/lib/i18n/locales";
+import { SUPPORTED_LOCALES, DEFAULT_LOCALE, isValidLocale } from "@/lib/i18n/locales";
 
 const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || "https://dev.manoonoils.com";
 
@@ -15,11 +15,12 @@ export async function generateMetadata({
   params: Promise<{ locale: string }>;
 }): Promise<Metadata> {
   const { locale } = await params;
-  const localePrefix = locale === "sr" ? "" : `/${locale}`;
+  const validLocale = isValidLocale(locale) ? locale : DEFAULT_LOCALE;
+  const localePrefix = validLocale === DEFAULT_LOCALE ? "" : `/${validLocale}`;
 
   const languages: Record<string, string> = {};
   for (const loc of SUPPORTED_LOCALES) {
-    const prefix = loc === "sr" ? "" : `/${loc}`;
+    const prefix = loc === DEFAULT_LOCALE ? "" : `/${loc}`;
     languages[loc] = `${baseUrl}${prefix}`;
   }
 

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;
+}