Have json/cfg return live config

Rather than reading the file off disk, have the json/cfg endpoint
return the live config from system state data.  This can improve
UI behaviour as it can never be out of date or include values that
do not apply to the current firmware install.

wled00/json.cpp

@@ -1028,7 +1028,7 @@ class LockedJsonResponse: public AsyncJsonResponse {
 void serveJson(AsyncWebServerRequest* request)
 {
   enum class json_target {
-    all, state, info, state_info, nodes, effects, palettes, fxdata, networks
+    all, state, info, state_info, nodes, effects, palettes, fxdata, networks, config
   };
   json_target subJson = json_target::all;
 
@@ -1041,6 +1041,7 @@ void serveJson(AsyncWebServerRequest* request)
   else if (url.indexOf(F("palx"))  > 0) subJson = json_target::palettes;
   else if (url.indexOf(F("fxda"))  > 0) subJson = json_target::fxdata;
   else if (url.indexOf(F("net"))   > 0) subJson = json_target::networks;
+  else if (url.indexOf(F("cfg"))   > 0) subJson = json_target::config;
   #ifdef WLED_ENABLE_JSONLIVE
   else if (url.indexOf("live")     > 0) {
     serveLiveLeds(request);
@@ -1051,9 +1052,6 @@ void serveJson(AsyncWebServerRequest* request)
     request->send_P(200, FPSTR(CONTENT_TYPE_JSON), JSON_palette_names);
     return;
   }
-  else if (url.indexOf(F("cfg")) > 0 && handleFileRead(request, F("/cfg.json"))) {
-    return;
-  }
   else if (url.length() > 6) { //not just /json
     serveJsonError(request, 501, ERR_NOT_IMPL);
     return;
@@ -1085,6 +1083,8 @@ void serveJson(AsyncWebServerRequest* request)
       serializeModeData(lDoc); break;
     case json_target::networks:
       serializeNetworks(lDoc); break;
+    case json_target::config:
+      serializeConfig(lDoc); break;
     case json_target::state_info:
     case json_target::all:
       JsonObject state = lDoc.createNestedObject("state");