Prevent Range Test from running on public/default channel

[dubsector]

Added logic in RangeTestConfigItemList.kt to detect when the primary channel is using the default/public PSK.

Disabled the entire Range Test configuration UI when the device is on a public channel.

Added a backend safety check to ensure the Range Test module cannot be enabled even if the UI is bypassed.

Automatically forces enabled = false in the module config when public channel conditions are detected.

Ensures behavior matches existing code style and Meshtastic’s definition of “public channel.”

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