Fix Ethernet init checking hardwareStatus before begin() and deduplicate CLI code

The Ethernet retry loop in repeater and room server checked
hardwareStatus() and linkStatus() before calling Ethernet.begin(),
which always returned EthernetNoHardware since hardware detection
only happens during begin(). Extract shared Ethernet CLI code into
EthernetCLI.h to prevent future divergence. Also fix time_t type
mismatch in companion radio Ethernet init.

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

examples/companion_radio/main.cpp

@@ -163,7 +163,7 @@ void setup() {
   the_mesh.startInterface(serial_interface);
 #elif defined(ETHERNET_ENABLED)
   Serial.print("Waiting for serial to connect...\n");
-  time_t timeout = millis();
+  unsigned long timeout = millis();
   while (!Serial) {
     if ((millis() - timeout) < 5000) { delay(100); } else { break; }
   }