[FEATURE] Enable Background Playback for Hands-Free Mode

[xenom34]

Describe the feature request

When the "Hands-Free" mode is activated, it should be possible to exit the application and continue using the device for other tasks. Currently, this is not the case. Hands-free mode is activated by long-pressing the central button on the screen in 'Memorize' mode.

Expected behavior

1. The user can activate "Hands-Free" mode and exit the app while the lesson continues to play.
2. An audio player should appear in the device's notification area, similar to when playing music, but instead, it's the Arutairu app using text-to-speech to pronounce lesson words.