Stats: Categorize Ecosia referrer as a Search Engine

Ecosia https://www.ecosia.org/ is becoming a very popular search engine, with its focus on both privacy and combating climate change.

Currently, it appears as a standard referrer, not a search engine:

It would be nice if it was categorized as a search engine along with the rest for better organization.

Start by locating the Stats referrer classification entry point and the existing search-engine definitions. Verify how Ecosia is currently categorized, then confirm that it appears with the other search engines in the relevant Stats view; the issue provides no file or test path.

Written by the indexing model from the issue text.