let the key K (karaoke mode) toggle through 2 or 3 states depending on user configured vocals volume

[dgruss]

Hopefully addresses #1360.

This PR:

• persists the user-selected vocals slider value as LastPartialVocalsVolume
• keeps K toggle vocals between 100% and 0% by default
• if the save partial volume is not 0 or 100, it uses a 3-step cycle, 100% -> saved partial volume -> 0% -> 100%
• makes sure K also works from the TAB popup and Sound Options screen, with visible slider updates
• applies the toggle at the settings level even when no karaoke/instrumental stream is active

Aufzeichnung.2026-07-05.220511.mp4