feat: show pub_key as base64 in discover detail screen

Replaces "Type: Repeater" line with the node's full public key encoded
as base64 (44 chars). drawTextEllipsized clips it with ... to fit the
128px wide display. Type is already visible in the inverted header.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/MyMesh.h

@@ -92,6 +92,7 @@ struct DiscoverResult {
   int8_t  rssi;       // RSSI of the response as received by us (dBm)
   int8_t  snr_x4;     // SNR of the response as received by us (dB × 4)
   int8_t  remote_snr_x4; // SNR at which responder heard our request (dB × 4)
+  uint8_t pub_key[PUB_KEY_SIZE];
   uint32_t timestamp;
 };