fix: ensure canonical URLs match actual URLs with locale prefix

- Update [slug]/page.tsx to always include locale in canonical URL
- Update by-oil/page.tsx to use consistent locale prefix
- Update by-concern/page.tsx to use consistent locale prefix
- All canonical URLs now match actual URLs: /{locale}/solutions/{slug}

src/app/[locale]/solutions/by-oil/page.tsx

@@ -19,7 +19,7 @@ export async function generateMetadata({
   const t = await getTranslations({ locale: validLocale, namespace: "Solutions.ByOil" });
   
   const baseUrl = process.env.NEXT_PUBLIC_SITE_URL || "https://manoonoils.com";
-  const localePrefix = validLocale === DEFAULT_LOCALE ? "/sr" : `/${validLocale}`;
+  const localePrefix = `/${validLocale}`;
   const canonicalUrl = `${baseUrl}${localePrefix}/solutions/by-oil`;
 
   return {