Fix signed radio provisioning metrics

[nilu96]

Summary

Fixes #101.

The LoRa provisioning metrics cast Noise Floor and Last RSSI to uint16_t, causing negative dBm values to wrap into large positive integers. For example, -36 became 65500.

This change:

• Preserves Noise Floor and Last RSSI as signed integer values.
• Exposes Last SNR as a signed floating-point metric, preserving negative and quarter-dB values.

Testing

• Built heltec_wifi_lora_32_V4-local successfully.