feat: implement locale-aware routing with [locale] dynamic segments

WARNING: This change breaks existing SEO URLs for Serbian locale.

Changes:
- Migrated from separate locale folders (src/app/en/, src/app/de/, etc.)
  to [locale] dynamic segments (src/app/[locale]/)
- Serbian is now at /sr/ instead of / (root)
- English at /en/, German at /de/, French at /fr/
- All components updated to generate locale-aware links
- Root / now redirects to /sr (307 temporary redirect)

SEO Impact:
- Previously indexed Serbian URLs (/, /products, /about, /contact)
  will now return 404 or redirect to /sr/* URLs
- This is a breaking change for SEO - Serbian pages should ideally
  remain at root (/) with only non-default locales getting prefix
- Consider implementing 301 redirects from old URLs to maintain
  search engine rankings

Technical Notes:
- next-intl v4 with [locale] structure requires ALL locales to
  have the prefix (cannot have default locale at root)
- Alternative approach would be separate folder structure per locale

src/app/[locale]/layout.tsx

@@ -0,0 +1,25 @@
+import { NextIntlClientProvider } from "next-intl";
+import { getMessages, setRequestLocale } from "next-intl/server";
+import { routing } from "@/i18n/routing";
+
+export function generateStaticParams() {
+  return routing.locales.map((locale) => ({ locale }));
+}
+
+export default async function LocaleLayout({
+  children,
+  params,
+}: {
+  children: React.ReactNode;
+  params: Promise<{ locale: string }>;
+}) {
+  const { locale } = await params;
+  setRequestLocale(locale);
+  const messages = await getMessages();
+
+  return (
+    <NextIntlClientProvider messages={messages}>
+      {children}
+    </NextIntlClientProvider>
+  );
+}