Disable Range Test module when primary channel is public/unsecured

[dubsector]

Updated RangeTestConfig.swift to determine whether the primary channel (index 0) is operating without encryption or with a 1-byte minimal PSK.

Disabled Range Test UI controls when on a public/default channel to prevent user interaction.

Added safety enforcement in the save operation: Range Test enabled flag is automatically forced to false before sending updates to the device.

Introduced a computed property isPrimaryChannelPublic following existing code patterns and security indicators (e.g., hexDescription PSK length).

Matches the behavior implemented in the Android client for consistent policy across platforms.

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