robustness: Rewind file pointer before writing initial data

if the presets.json file initially contains ``{}`` (valid JSON, could be created by user edit), we need to first rewind the file. Otherwise the result would be ``{}{"0":{}}`` (ivalid JSON)

wled00/file.cpp

@@ -188,6 +188,7 @@ static bool appendObjectToFile(const char* key, const JsonDocument* content, uin
   if (f.size() < 3) {
     char init[10];
     strcpy_P(init, PSTR("{\"0\":{}}"));
+    f.seek(0, SeekSet);           // rewind to ensure we overwrite from the start, instead of appending
     f.print(init);
   }