Add silence-based stop for audio sync transmission

[netmindz]

This pull request adds a new feature to the audio synchronization system: the ability for the sender to automatically stop transmitting UDP audio sync packets after 30 seconds of continuous silence. This is configurable and exposed in the sync settings. The implementation tracks silence duration and skips transmission when appropriate, and the new setting is included in both the configuration and the user interface.

This very useful when you have one (or more) line-in setups combined with devices with their own mic set to "receive or local"

Audio sync silence stop feature:

• Added a new static boolean audioSyncSilenceStop to control whether the sender should stop transmitting UDP packets after 30 seconds of silence.
• Introduced a syncSilenceStart variable to track when continuous silence begins on the sender.
• Updated the UDP transmit logic to skip sending data if silence has persisted for more than 30 seconds and the feature is enabled.

Configuration and UI updates:

• Included the silence_stop parameter in the sync configuration JSON for both saving and loading settings. [1] [2]
• Added a dropdown for the new silence stop feature in the sync settings UI, allowing users to enable or disable the behavior.