minor improvements for debugging

* Info page: status of Serial (can be disconnected on new MCUs), and owner of RX/TX pins
* improved handling of Serial (don't write if not connected)
* added min available stack size to debug output (esp32 only)
* webserver show remaining stack size
* webserver: show string buffer usage

wled00/json.cpp

@@ -972,6 +972,12 @@ void serializeInfo(JsonObject root)
     default: root[F("e32flashtext")] = F(" (other)"); break;
   }
   #endif
+  #if defined(WLED_DEBUG) || defined(WLED_DEBUG_HOST) || defined(SR_DEBUG) || defined(SR_STATS)
+  // WLEDMM add status of Serial, incuding pin alloc
+  root[F("serialOnline")] = Serial ? (canUseSerial()?F("Serial ready"):F("Serial in use")) : F("Serial disconected");  // "Disconnected" may happen on boards with USB CDC
+  root[F("sRX")] = pinManager.isPinAllocated(hardwareRX) ? pinManager.getPinOwnerText(hardwareRX): F("free");
+  root[F("sTX")] = pinManager.isPinAllocated(hardwareTX) ? pinManager.getPinOwnerText(hardwareTX): F("free");
+  #endif
   // end WLEDMM
 
   root[F("uptime")] = millis()/1000 + rolloverMillis*4294967;