fix: propagate errors from DuckDuckGo search and GetResults

- Change SearchPage.GetResults() to return ([]Result, error) so ForEach
  errors are no longer silently discarded
- Fix Search() to return the ForEach error instead of nil
- Update cmd caller to check GetResults() errors

Closes #5, #6

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sites/duckduckgo/duckduckgo.go

@@ -130,5 +130,5 @@ func (c Config) Search(ctx context.Context, b extractor.Browser, query string) (
 		return nil
 	})
 
-	return res, nil
+	return res, err
 }