Fix GPS UART consuming +8mA when disabled (nRF52)

When GPS is disabled (or not detected), Serial1.begin() was called
during initBasicGPS() but never ended. On nRF52840 the UARTE
peripheral stays active, drawing ~8mA through the peripheral itself
and current leaking via TX pin into the GPS module's ESD diodes.

- Call Serial1.end() in initBasicGPS() and stop_gps() to disable UART
- Re-initialize Serial1 in start_gps() so GPS can be toggled back on
- Gate _location->loop() behind gps_active to prevent accessing ended
  Serial (and avoid pointless NMEA polling when GPS is off)

Fixes #1628

Author: weebl2000

src/helpers/sensors/EnvironmentSensorManager.cpp

@@ -583,6 +583,7 @@ void EnvironmentSensorManager::initBasicGPS() {
     MESH_DEBUG_PRINTLN("No GPS detected");
   }
   _location->stop();
+  Serial1.end();  // Disable UART peripheral to save power (significant on nRF52)
   gps_active = false; //Set GPS visibility off until setting is changed
 }
 
@@ -680,6 +681,14 @@ void EnvironmentSensorManager::start_gps() {
     return;
   #endif
 
+  // Re-initialize UART (was closed by stop_gps() to save power)
+  Serial1.setPins(PIN_GPS_TX, PIN_GPS_RX);
+  #ifdef GPS_BAUD_RATE
+  Serial1.begin(GPS_BAUD_RATE);
+  #else
+  Serial1.begin(9600);
+  #endif
+
   _location->begin();
   _location->reset();
 
@@ -697,6 +706,7 @@ void EnvironmentSensorManager::stop_gps() {
   #endif
 
   _location->stop();
+  Serial1.end();  // Disable UART peripheral to save power (significant on nRF52)
 
   #ifndef PIN_GPS_EN
   MESH_DEBUG_PRINTLN("Stop GPS is N/A on this board. Actual GPS state unchanged");
@@ -707,7 +717,9 @@ void EnvironmentSensorManager::loop() {
   static long next_gps_update = 0;
 
   #if ENV_INCLUDE_GPS
-  _location->loop();
+  if (gps_active) {
+    _location->loop();
+  }
   if (millis() > next_gps_update) {
 
     if(gps_active){