feat(proxy): add error mapper for HTTP status code mapping

- Add error_mapper.rs module to map ProxyError to HTTP status codes
- Implement map_proxy_error_to_status() for error classification
- Implement get_error_message() for user-friendly error messages
- Support all error types: upstream, timeout, connection, provider failures
- Include comprehensive unit tests for all mappings

src-tauri/src/proxy/mod.rs

@@ -4,6 +4,7 @@
 
 pub mod circuit_breaker;
 pub mod error;
+pub mod error_mapper;
 mod forwarder;
 mod handlers;
 mod health;