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/data/index.js

@@ -676,6 +676,8 @@ function populateInfo(i)
 ${urows}
 ${urows===""?'':'<tr><td colspan=2><hr style="height:1px;border-width:0;color:SeaGreen;background-color:Seagreen"></td></tr>'}
 ${i.opt&0x100?inforow("Net Serial ☾","<button class=\"btn btn-xs\" onclick=\"requestJson({'netDebug':"+(i.opt&0x0080?"false":"true")+"});\"><i class=\"icons "+(i.opt&0x0080?"on":"off")+"\">&#xe08f;</i></button>"):''}
+${i.serialOnline?inforow(i.serialOnline,"TX="+i.sTX,"; RX="+i.sRX):""}
+${i.opt&0x100?'<tr><td colspan=2><hr style="height:1px;border-width:0;color:GoldenRod;background-color:GoldenRod"></td></tr>':''}
 ${inforow("Build",i.vid)}
 ${inforow("Estimated current",pwru)}
 ${inforow("Average FPS",i.leds.fps)}