Add support for forwarding `security-key` and `x-security-key` headers in the
proxy route. The proxy will now automatically inject a security key from
environment variables if it is not already present in the request headers.
Additionally, introduce new test API pages to facilitate debugging and
verifying API responses, including localized country name resolution and
proxy functionality.
- Add `security-key` and `x-security-key` to forwarded headers in proxy
- Implement automatic injection of security key in `src/app/api/proxy/route.ts`
- Add `src/app/[lang]/test-api/page.tsx` for comprehensive API testing
- Add `src/app/test-api/page.tsx` as a root-level redirect/wrapper for the
test API page