Replace INPUT_PULLUP with PSEL loopback to fix Serial1.end() hang

The internal pullup (~13kΩ) wasn't strong enough to overcome the GPS
module's TX line pulling LOW through ESD diodes when powered off.

Instead, redirect UARTE0 PSEL.RXD to its own TXD pin before calling
Serial1.end(). The UARTE drives TX HIGH in idle state, so RX reads a
guaranteed idle line regardless of GPS module pin state. RXTO fires
promptly and end() completes without hanging.

Author: weebl2000

src/helpers/sensors/EnvironmentSensorManager.cpp

@@ -583,10 +583,13 @@ void EnvironmentSensorManager::initBasicGPS() {
     MESH_DEBUG_PRINTLN("No GPS detected");
   }
   _location->stop();
-  // Pull up MCU's RX pin before ending UART. When GPS powers down, its TX
-  // line drops LOW (break condition) which prevents nRF52 UARTE's RXTO event
-  // from firing, causing Serial1.end() to hang indefinitely.
-  pinMode(PIN_GPS_TX, INPUT_PULLUP);  // PIN_GPS_TX = toward CPU = MCU's RX
+#ifdef NRF52_PLATFORM
+  // Redirect UART RX to its own TX pin before ending Serial. When GPS powers
+  // down its TX drops LOW (break condition), preventing nRF52 UARTE's RXTO
+  // event from firing and hanging Serial1.end() indefinitely. Looping RX to
+  // TX guarantees an idle (HIGH) line so RXTO fires promptly.
+  NRF_UARTE0->PSEL.RXD = NRF_UARTE0->PSEL.TXD;
+#endif
   Serial1.end();  // Disable UART peripheral to save power (significant on nRF52)
   gps_active = false; //Set GPS visibility off until setting is changed
 }
@@ -710,8 +713,10 @@ void EnvironmentSensorManager::stop_gps() {
   #endif
 
   _location->stop();
-  // Pull up MCU's RX pin before ending UART - see comment in initBasicGPS()
-  pinMode(PIN_GPS_TX, INPUT_PULLUP);
+#ifdef NRF52_PLATFORM
+  // Redirect UART RX to TX pin — see comment in initBasicGPS()
+  NRF_UARTE0->PSEL.RXD = NRF_UARTE0->PSEL.TXD;
+#endif
   Serial1.end();  // Disable UART peripheral to save power (significant on nRF52)
 
   #ifndef PIN_GPS_EN