Address PR review feedback for Ethernet support

- Add #pragma once to SerialEthernetInterface.h
- Rename TCP_PORT to ETH_TCP_PORT with #ifndef guard
- Fix typos: "initalizing" → "initializing"
- Fix #elif without condition → #else for STM32 block
- Replace infinite loop on ETH init failure with halt()
- Remove heartbeat Serial.print(".") output
- Remove dead beginETH() call (ETH_ENABLED, not RAK_ETH_ENABLE)
- Comment out MESH_DEBUG and ETH_DEBUG_LOGGING build flags

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

src/helpers/nrf52/SerialEthernetInterface.cpp

@@ -20,7 +20,7 @@ SPIClass ETH_SPI_PORT(NRF_SPIM1, PIN_SPI1_MISO, PIN_SPI1_SCK, PIN_SPI1_MOSI);
 
 bool SerialEthernetInterface::begin() {
   
-  ETH_DEBUG_PRINTLN("Ethernet initalizing");
+  ETH_DEBUG_PRINTLN("Ethernet initializing");
 
 #ifdef PIN_ETH_POWER_EN
         ETH_DEBUG_PRINTLN("Ethernet power enable");
@@ -90,7 +90,7 @@ bool SerialEthernetInterface::begin() {
   ETH_DEBUG_PRINT_IP("Gateway", gateway);
 
   server.begin();   // start listening for clients
-  ETH_DEBUG_PRINTLN("Ethernet: listening on TCP port: %d", TCP_PORT);
+  ETH_DEBUG_PRINTLN("Ethernet: listening on TCP port: %d", ETH_TCP_PORT);
 
   return true;
 }