LoRa Scanner: MeshCore by default, and its public channel read

The MeshCore preset is the default for a new device, in the Settings and
in the radio before the Scanner is opened. The setting now accepts it:
it only took the 7 Meshtastic presets, so MeshCore picked in the App was
not saved.

Messages on MeshCore's public channel are decrypted with the channel's
published key (AES-128, checked with 2 bytes of HMAC-SHA256): the row
shows who says they sent it and the start of the text, the details the
sender, the time and the whole text. Other channels and private messages
stay encrypted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

src/services/radio_service.cpp

@@ -39,7 +39,7 @@ constexpr float kDeafBelow = -108.0f;
 RadioService* RadioService::instance_ = nullptr;
 
 RadioService::RadioService() : lock_(xSemaphoreCreateMutex()) {
-    lora::ScanPreset p = lora::scanPreset(0);  // LongFast (Q95)
+    lora::ScanPreset p = lora::scanPreset(lora::kDefaultScanPreset);
     config_ = {p.frequencyHz, p.bwKHz, p.sf, p.cr, p.syncWord, p.preamble, p.name};
 }