Add Frequency Scale to Spectrograms in Review Mode

[mariaptoro]

As an active BirdNET Analyzer user and validator, I would find it extremely useful to have a frequency scale (y-axis) displayed in Review mode spectrograms.

For closely related or congeneric species, small frequency differences are often critical for accurate validation. A visible frequency reference would make manual review more precise and consistent, especially in species-rich tropical soundscapes.

This seems like a relatively small UI enhancement with a significant benefit for users who spend time validating detections.

[Josef-Haupt]

yes, makes sense