Add hybrid squelch UI and manual threshold control feature

[xnetinho]

This pull request enhances the squelch functionality in the audio settings by introducing automatic and manual squelch modes, along with a configurable squelch threshold. These changes improve user control and flexibility over audio filtering, and update both the UI and backend logic to support the new features.

Audio squelch feature enhancements:

• Added squelchAuto and squelchLevel properties to the AudioUiSettings type to support automatic (statistical) and manual squelch modes, with a user-defined threshold in dB.
• Updated the state initialization in App to include squelchAuto (default: true) and squelchLevel (default: -80).

UI improvements:

• Modified AudioPanel to display toggles for squelch auto/manual mode and a slider for manual squelch level when squelch is enabled, allowing users to easily adjust squelch settings.

Backend logic updates:

• Enhanced the squelch effect in useAudioClient to send both the squelch mode and level to the backend, and track changes using a composite key to prevent redundant updates.
• Changed the reference tracking for squelch state in useAudioClient from a boolean to a string key to accommodate the new squelch properties.