fix: canonical URLs to always include locale prefix

Fixed Canonical Redirect Error in Google Search Console by ensuring
canonical URLs always include the locale prefix (/sr, /en, /de, /fr)
instead of omitting it for the default locale.

Changes:
- layout.tsx: Fixed canonical and hreflang URL generation
- page.tsx: Fixed homepage canonical URL
- contact/page.tsx: Fixed contact page canonical URL
- about/page.tsx: Fixed about page canonical URL
- products/page.tsx: Fixed products page canonical URL
- products/[slug]/page.tsx: Fixed product detail canonical URL

Before: /contact (when locale=sr)
After: /sr/contact (all locales)

Fixes: Canonical Redirect Error in Google Search Console

src/app/[locale]/about/page.tsx

@@ -19,7 +19,7 @@ export async function generateMetadata({ params }: AboutPageProps): Promise<Meta
   const metadata = getPageMetadata(validLocale as Locale);
   const keywords = getPageKeywords(validLocale as Locale, 'about');
   
-  const localePrefix = validLocale === DEFAULT_LOCALE ? "" : `/${validLocale}`;
+  const localePrefix = `/${validLocale}`;
   const canonicalUrl = `${baseUrl}${localePrefix}/about`;
   
   return {