Fix dangling pointer bug in URL storage

The previous implementation stored parsed_url which contained pointers
into the user's string literal that could be invalidated. This fix:

1. Allocates a copy of the URL string (url_buffer)
2. Parses the copy so all pointers reference owned memory
3. Removes redundant domain and path fields from HTTP_ClientConn
4. Updates save_cookies to use url.authority.host.text and url.path
5. Updates cleanup code to free url_buffer instead of domain.ptr

This ensures the URL and all its components remain valid for the
lifetime of the connection.

src/client.h

@@ -75,12 +75,10 @@ typedef struct {
     // TODO: comment
     bool trace_bytes;
 
-    // Endpoint domain and path. These must be saved
-    // in case the server wants to set some cookies.
-    HTTP_String domain;
-    HTTP_String path;
-
     // Parsed URL for connection establishment
+    // The url_buffer contains the allocated copy of the URL string,
+    // and url.* pointers reference into this buffer
+    char *url_buffer;
     HTTP_URL url;
 
     // Data received from the server