updated on ()
- improved smart-selection buttons (disabled states and clearer messages)
- added global random selection across all pages in the filtered level
- new end-of-series feedback with direct actions: Retry this series, New series, Smart selection
- fixed smart selection: prioritizes confusions (target species + confused species), then Difficult/To improve
- validates playable sounds before starting a learning series
- added continent/country/source filters in "species to work on" statistics
- fixed UTF-8 encoding in the species search engine